cmd/internal/obj, cmd/link: fix st_other field on PPC64

In PPC64 ELF files, the st_other field indicates the number of
prologue instructions between the global and local entry points.
We add the instructions in the compiler and assembler if -shared is used.
We were assuming that the instructions were present when building a
c-archive or PIE or doing dynamic linking, on the assumption that those
are the cases where the go tool would be building with -shared.
That assumption fails when using some other tool, such as Bazel,
that does not necessarily use -shared in exactly the same way.

This CL records in the object file whether a symbol was compiled
with -shared (this will be the same for all symbols in a given compilation)
and uses that information when setting the st_other field.

Fixes #20290.

Change-Id: Ib2b77e16aef38824871102e3c244fcf04a86c6ea
Reviewed-on: https://go-review.googlesource.com/43051
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Hudson-Doyle <michael.hudson@canonical.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/internal/obj/objfile.go

@@ -338,6 +338,9 @@ func (w *objWriter) writeSym(s *LSym) {
 	if s.ReflectMethod() {
 		flags |= 1 << 2
 	}
+	if ctxt.Flag_shared {
+		flags |= 1 << 3
+	}
 	w.writeInt(flags)
 	w.writeInt(int64(len(s.Func.Autom)))
 	for _, a := range s.Func.Autom {