compiler: use GMP_RNDN rather than MPFR_RNDN
compiler: use GMP_RNDN rather than MPFR_RNDN

This fixes the build with mpfr 2.4.2.

Fixes https://gcc.gnu.org/PR89598

Change-Id: Ifce8ac7d9635c3deb3bb43ec3ca0d32a1bf6d352
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/165418
Reviewed-by: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>

golang/gofrontend,golang/gofrontend,golang/gofrontend,golang/gofrontend