compiler: cleanups permitted by GCC requirement of MPFR 3.1.0

For MPFR functions, change from GMP_RND* to MPFR_RND*.
Also change mp_exp_t to mpfr_expt_t.

Fixes https://gcc.gnu.org/PR92463

Change-Id: I542f09fd7a3338318e8dc4367cf3f515b9eac59a
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/216417
Reviewed-by: Cherry Zhang <cherryyz@google.com>
3 files changed