[RISCV][NFC] Replace hard-coded CSR duplication with symbolic references

Reviewers: asb, lenary

Reviewed By: asb, lenary

Subscribers: MaskRay, hiraditya, rbar, johnrusso, simoncook, apazos, sabuasal, niosHD, kito-cheng, shiva0217, zzheng, edward-jones, rogfer01, MartinMosbeck, brucehoult, the_o, rkruppe, PkmX, jocewei, psnobl, benna, Jim, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64139

Patch by James Clarke (jrtc27)

llvm-svn: 365195