msun/riscv: expose fe{disable,enable}except

This is required for GCC to build.

PR: 272759
Reported by: dgilbert@eicat.ca
Submitted by: jrtc27
Differential Revision: https://reviews.freebsd.org/D44333

(cherry picked from commit 1947a9383ec3a048e334022365aa199a6ae55289)