[RISCV] Refactor GPRF64 register class to make it usable for Zacas. (#77408)

-Rename to GPRPair.
-Rename registers to be named like X10_X11 instead of X10_PD. Except X0
 which is now X0_Pair since it is not paired with X1.
-Use unknown size and offset for the subreg indices. This might
 be a functional change, but does not affect any lit tests.