[SPARC] Add GNU syntax extensions of WRPR instruction

Add two GNU extension forms of wrpr:

wrpr %reg, %rd -> wrpr %reg, %g0, %rd
wrpr  imm, %rd -> wrpr  %g0, imm, %rd

Note: since the semantics of wrpr %rs1, %rs2/imm, %rd is %rd <- (%rs1 xor %rs2/imm),
in case (1) it technically doesn't matter whether the %g0 is placed as the first or
second operand, however, I'm placing it as the second operand to match the encoding
of GNU as.

This fixes the wrpr instruction part of bug #40792

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D139535