cmd/internal/goobj: fix buglet in object file reader

The code in the new (introduced in 1.15) Go object file reader was
casting a pointer-mmaped-memory into a large array prior to performing
a read of the relocations section:

	return (*[1<<20]Reloc)(unsafe.Pointer(&r.b[off]))[:n:n]

For very large object files, this artificial array isn't large enough
(that is, there are more than 1048576 relocs to read), so update the
code to use a larger artifical array size.

Fixes #41621.

Change-Id: Ic047c8aef4f8a3839f2e7e3594bce652ebd6bd5b
Reviewed-on: https://go-review.googlesource.com/c/go/+/278492
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Trust: Than McIntosh <thanm@google.com>

src/cmd/internal/goobj/objfile.go

@@ -483,6 +483,11 @@ func (r *RefFlags) SetFlag2(x uint8) { r[9] = x }
 
 func (r *RefFlags) Write(w *Writer) { w.Bytes(r[:]) }
 
+// Used to construct an artifically large array type when reading an
+// item from the object file relocs section or aux sym section (needs
+// to work on 32-bit as well as 64-bit). See issue 41621.
+const huge = (1<<31 - 1) / RelocSize
+
 // Referenced symbol name.
 //
 // Serialized format:
@@ -792,7 +797,7 @@ func (r *Reader) Reloc(i uint32, j int) *Reloc {
 func (r *Reader) Relocs(i uint32) []Reloc {
 	off := r.RelocOff(i, 0)
 	n := r.NReloc(i)
-	return (*[1 << 20]Reloc)(unsafe.Pointer(&r.b[off]))[:n:n]
+	return (*[huge]Reloc)(unsafe.Pointer(&r.b[off]))[:n:n]
 }
 
 // NAux returns the number of aux symbols of the i-th symbol.
@@ -818,7 +823,7 @@ func (r *Reader) Aux(i uint32, j int) *Aux {
 func (r *Reader) Auxs(i uint32) []Aux {
 	off := r.AuxOff(i, 0)
 	n := r.NAux(i)
-	return (*[1 << 20]Aux)(unsafe.Pointer(&r.b[off]))[:n:n]
+	return (*[huge]Aux)(unsafe.Pointer(&r.b[off]))[:n:n]
 }
 
 // DataOff returns the offset of the i-th symbol's data.