runtime: add ppc and s390 to 32 build constraints for gccgo
Theses are not supported by the go compiler but it may helps porting to gccgo.
I have no idea if this change is correct, but it is weird that
os_linux32.go and os_linux64.go should have ppc & s390 but not all other
files gated to 32bits in the same package.
Change-Id: I0bb70cdb88c19096386320d02d546942263e009d
Reviewed-on: https://go-review.googlesource.com/c/go/+/714082 Reviewed-by: Ian Lance Taylor <iant@golang.org> Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Jorropo <jorropo.pgm@gmail.com> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>