[1/8][RISCV] Add rounding mode control variant for vfsub, vfrsub

Depends on D152996.

This patch-set aims to add a variant for the RVV floating-point
intrinsics that controls the rounding mode (`frm`). The rounding mode
variant appends `_rm` before the policy suffix to distinguish from
those without them.

Specification PR: riscv-non-isa/rvv-intrinsic-doc#226

This is the 1st patch of the patch-set.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D154628