Same as CL 34379, SyscallNoError and RawSyscallNoError are redeclared
for gccgo builds since CL 85756. Thus, move the declarations to a gc
specific file.

Updates golang/go#18312

Change-Id: Ia8f42b31e61a80530e5e90a44c0af32fd3452612
Reviewed-on: https://go-review.googlesource.com/91615
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>