compiler: use builtin memcmp directly

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.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/183537

From-SVN: r272620