[RISCV] Cleanup up the spec version references around fmaxnum/fminnum.

This previously made references to 2.3-draft which was a short
lived version number in 2017. It was replaced by date based
versions leading up to ratification.

This patch uses the latest ratified version number and just says
what the behavior is. Nothing here is in flux.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D100878