libgo: only build x/sys/cpu/cpu_gccgo.c on x86 systems

The C file has a build tag, but the procedure we use for building C
files ignores build tags.

This should fix the libgo build on non-x86 systems.

Change-Id: Ibb085930ba212c0d526ba0bc618a05f217f22f97
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/194378
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
4 files changed