cmd/compile/internal/gc: move intLiteral to gc.Node

intLiteral is used by the gins wrappers in arm64, ppc64 and
mips64. Refactor the function to a method on gc.Node and update
the callers to use the common copy.

Change-Id: I2db90d801a9cb18f8526eb921e13daa75ca1cf6f
Reviewed-on: https://go-review.googlesource.com/14744


Reviewed-by: Aram Hăvărneanu <aram@mgk.ro>
Reviewed-by: Dave Cheney <dave@cheney.net>
Run-TryBot: Dave Cheney <dave@cheney.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>