cmd/go: use gccSupportsFlag for -fsplit-stack

Don't assume that all (or only) 386/amd64 compilers support
-fsplit-stack.

Change-Id: Id0377020fddeaaf307757208486c42fe6850e070
Reviewed-on: https://go-review.googlesource.com/48592
Reviewed-by: Than McIntosh <thanm@google.com>
1 file changed