These functions are redeclared in gccgo builds, so move them
to separate files with the "!gccgo" build constraint.

Fixes golang/go#18312

Change-Id: I868cfb77fc394e5e96cee78af0f3abf43db6082a
Reviewed-on: https://go-review.googlesource.com/34379Reviewed-by: Ian Lance Taylor <iant@golang.org>