compiler: use builtin memcmp directly

Instead of going through a C function __go_memcmp, we can just
use __builtin_memcmp directly. This allows more optimizations in
the compiler backend.

Change-Id: I1e3497a64c3d325407c83479bdb4880f7f3885c3
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/183537
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>