README: Update s390 as supported and backporting info

* Add s390 as supported.
* Add backporting information for the distros.

Signed-off-by: Sumanth Korikkar <sumanthk@linux.ibm.com>

README.md

@@ -41,7 +41,7 @@ Supported Architectures
 - [x] x86-64
 - [x] ppc64le
 - [ ] arm64
-- [ ] s390
+- [x] s390 [upstream prerequisites](doc/s390-upstream-prerequisites.md)
 
 Installation
 ------------

doc/s390-upstream-prerequisites.md

@@ -0,0 +1,33 @@
+### s390 backporting
+
+**Prerequisite gcc patches (all backported to releases/gcc-11 branch):**
+- gcc-mirror/gcc@a1c1b7a IBM Z: Define NO_PROFILE_COUNTERS
+- gcc-mirror/gcc@0990d93 IBM Z: Use @PLT symbols for local functions in 64-bit mode
+- gcc-mirror/gcc@935b522 S/390: New option -mpic-data-is-text-relative
+- gcc-mirror/gcc@8753b13 IBM Z: fix section type conflict with -mindirect-branch-table
+
+**Prerequisite kernel patches:**
+**v5.19:**
+- 69505e3d9a39 bug: Use normal relative pointers in 'struct bug_entry'
+
+**v5.18:**
+- 602bf1687e6f s390/nospec: align and size extern thunks
+- 1d2ad084800e s390/nospec: add an option to use thunk-extern
+- eed38cd2f46f s390/nospec: generate single register thunks if possible
+- 2268169c14e5 s390: remove unused expoline to BC instructions
+- f0003a9e4c18 s390/entry: remove unused expoline thunk
+
+**v5.16:**
+- torvalds/linux@f6ac18f sched: Improve try_invoke_on_locked_down_task()
+- torvalds/linux@9b3c4ab sched,rcu: Rework try_invoke_on_locked_down_task()
+- torvalds/linux@00619f7 sched,livepatch: Use task_call_func()
+- torvalds/linux@8850cb6 sched: Simplify wake_up_*idle*()
+- torvalds/linux@5de62ea sched,livepatch: Use wake_up_if_idle()
+- torvalds/linux@96611c2 sched: Improve wake_up_all_idle_cpus() take #2
+
+**v5.15**
+- torvalds/linux@de5012b s390/ftrace: implement hotpatching
+- torvalds/linux@67ccddf ftrace: Introduce ftrace_need_init_nop()
+
+**v5.14:**
+- torvalds/linux@7561c14 s390/vdso: add .got.plt in vdso linker script