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 <thanm@google.com>
1 file changed