[llvm] [RISC-V][MC] Accept an absolute variable value as a CSR number (PR #67377)

[Jessica Clarke via llvm-commits]

jrtc27 wrote:

What I don't like about this syntax is it introduces parsing ambiguity if you do, say, `.set fflags, 42; csrr a0, fflags`; should that use fflags-the-CSR or fflags-the-constant?

https://github.com/llvm/llvm-project/pull/67377