Partially revert patch that made VXRM a global register on RISC-V

Partially revert patch that made VXRM a global register on RISC-V

This is something that fell through the cracks in gcc-15.  VXRM isn't heavily
used, so errors in this space could easily be going unnoticed right now.

Essentially we made VXRM a global register a while back, it was done somewhat
speculatively as we didn't have a case where it really mattered.  Richard S.
then argued the patch was wrong and I agreed with him, but never got around to
reverting the hunk in question.

So that's what I'm finally doing here.  Note that I kept the tests from the
patch which made VXRM a global.  Those should continue to work.

Bootstrapped and regression tested on a BPI & Pioneer system and checked on
riscv{32,64}-elf as well.

gcc/
	* config/riscv/riscv.cc (riscv_conditional_register_usage): Revert
	patch that made VXRM a global register.