compiler: use GMP_RNDN rather than MPFR_RNDN

Missed one last time around.  This fixes the build with mpfr 2.4.2.

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

Change-Id: Id866ea8f1cac29525f0bdbab073e1d65bd3df686
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/165420
Reviewed-by: Cherry Zhang <cherryyz@google.com>
1 file changed