[dev.link] cmd/compile: make compiler-generated ppc64 TOC symbols static

[dev.link] cmd/compile: make compiler-generated ppc64 TOC symbols static

Set the AttrStatic flag on compiler-emitted TOC symbols for ppc64; these
symbols don't need to go into the final symbol table in Go binaries.
This fixes a buglet introduced by CL 240539 that was causing failures
on the aix builder.

Change-Id: If8b63bcf6d2791f1ec5a0c371d2d11e806202fd2
Reviewed-on: https://go-review.googlesource.com/c/go/+/241637
Run-TryBot: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>