Add complex labeling scheme for landing pad

Function signature based labeling scheme, follow the "Function types" mangling
rule defeind in Itanium C++ ABI.

With few specific rules for member functions:

- Member functions  use the "Pointer-to-member types" mangling rule
- Virtual functions use the member function type of the base
  class that first defined the virtual function.

riscv-elf.adoc

@@ -662,6 +662,7 @@ using all other PLT sytle.
 |===
 | Default PLT                  | -
 | Simple landing pad PLT | Must use this PLT style when `GNU_PROPERTY_RISCV_FEATURE_1_CFI_LP_SIMPLE` is set.
+| Complex landing pad PLT | Must use this PLT style when `GNU_PROPERTY_RISCV_FEATURE_1_CFI_LP_COMPLEX` is set.
 |===
 
 The first entry of a shared object PLT is a special entry that calls
@@ -673,6 +674,9 @@ dynamic linker before the executable is started. Lazy resolution of GOT
 entries is intended to speed up program loading by deferring symbol
 resolution to the first time the function is called.
 
+The PLT entry is 16 bytes for the default PLT style and the simple landing pad
+PLT style, and 32 bytes for the complex landing pad PLT style.
+
 The first entry in the PLT occupies two 16 byte entries for the default PLT style:
 
 [,asm]
@@ -704,7 +708,25 @@ And occupies three 16 byte entries for the simple landing pad PLT style:
     nop
 ----
 
-Subsequent function entry stubs in the PLT take up 16 bytes.
+The complex landing pad PLT style occupies two 32 byte entries:
+
+[,asm]
+----
+1:  lpad 0
+    auipc  t2, %pcrel_hi(.got.plt)
+    sub    t1, t1, t3               # shifted .got.plt offset + hdr size + 24
+    l[w|d] t3, %pcrel_lo(1b)(t2)    # _dl_runtime_resolve
+    addi   t1, t1, -(hdr size + 24) # shifted .got.plt offset
+    addi   t0, t2, %pcrel_lo(1b)    # &.got.plt
+    srli   t1, t1, log2(32/PTRSIZE) # .got.plt offset
+    l[w|d] t0, PTRSIZE(t0)          # link map
+    jr     t3
+    nop
+    nop
+----
+
+Subsequent function entry stubs in the PLT take up 16 bytes or 32 bytes depends
+on the style.
 On the first call to a function, the entry redirects to the first PLT entry
 which calls `_dl_runtime_resolve` and fills in the GOT entry for subsequent
 calls to the function.
@@ -727,6 +749,19 @@ The code sequences of the PLT entry for the the simple landing pad PLT style:
     jalr    t1, t3
 ----
 
+The code sequences of the PLT entry for the the complex landing pad PLT style:
+[,asm]
+----
+1:  lpad    <hash-value-for-function>
+    auipc   t3, %pcrel_hi(function@.got.plt)
+    l[w|d]  t3, %pcrel_lo(1b)(t3)
+    lui     t2, <hash-value-for-function>
+    jalr    t1, t3
+    nop
+    nop
+    nop
+----
+
 ==== Procedure Calls
 
 `R_RISCV_CALL` and `R_RISCV_CALL_PLT` relocations are associated with
@@ -1469,6 +1504,7 @@ a different features.
 | Bit | Bit Name
 |   0 | GNU_PROPERTY_RISCV_FEATURE_1_CFI_LP_SIMPLE
 |   1 | GNU_PROPERTY_RISCV_FEATURE_1_CFI_SS
+|   2 | GNU_PROPERTY_RISCV_FEATURE_1_CFI_LP_COMPLEX
 |===
 
 `GNU_PROPERTY_RISCV_FEATURE_1_CFI_LP_SIMPLE` This bit indicate that all executable
@@ -1486,6 +1522,12 @@ compressed instructions then loading an executable with this bit set requires
 the execution environment to provide the `Zicfiss` extension or to provide both
 the `Zcmop` and `Zimop` extensions.
 
+`GNU_PROPERTY_RISCV_FEATURE_1_CFI_LP_COMPLEX` This bit indicate that all executable
+sections are built to be compatible with the landing pad mechanism provided by
+the `Zicfilp` extension. An executable or shared library with this bit set is
+required to generate PLTs with the landing pad (`lpad`) instruction, and all
+label are set to a value which hashed from its function signature.
+
 === Mapping Symbol
 
 The section can have a mixture of code and data or code with different ISAs.
@@ -1529,6 +1571,61 @@ attribute is recording for minimal execution environment requirements, so the
 ISA information from arch attribute is not enough for the disassembler to
 disassemble the `rv64gcv` version correctly.
 
+== Label Value Compuatation for Complex Labeling Scheme Landing Pad
+
+The label value for the complex labeling scheme landing pad is computed from the
+hash of the function signature string, which uses the same scheme as the
+"Function types" mangling rule defined in the _Itanium {Cpp} ABI_
+<<itanium-cxx-abi>>, the value is taken from the lower 20 bits of the MD5
+hash result of the function signature string.
+
+Additionally, here are a few specific rules for {Cpp} member functions:
+
+- {Cpp} member functions should use the "Pointer-to-member types" mangling rule
+  defined in the _Itanium {Cpp} ABI_ <<itanium-cxx-abi>>.
+- Virtual functions in {Cpp} should use the member function type of the base
+  class that first defined the virtual function.
+
+
+Example:
+
+[,cxx]
+----
+
+double foo(int, float *);
+
+class Base
+{
+public:
+  virtual void memfunc1();
+  virtual void memfunc2(int);
+};
+
+class Derived : public Base
+{
+public:
+  virtual void memfunc1();
+  virtual void memfunc3(double);
+  void memfunc4();
+};
+
+class DerivedDerived : public Derived
+{
+public:
+  virtual void memfunc2(int);
+  virtual void memfunc3(double);
+};
+
+----
+
+The function signatures for the above functions are described below:
+
+- `foo` is encoded as `FdiPfE`.
+- `Base::memfunc1` and `Derived::memfunc1` are both encoded as `M4BaseFvvE`.
+- `Base::memfunc2` and `DerivedDerived::memfunc2` are both encoded as `M4BaseFviE`.
+- `Derived::memfunc3` and `DerivedDerived::memfunc3` are both encoded as `M7DerivedFvdE`.
+- `Derived::memfunc4` is encoded as `M7DerivedFvvE`.
+
 == Linker Relaxation
 
 At link time, when all the memory objects have been resolved, the code sequence