[RISCV] Allow parsing dot '.' in assembly

Summary:
Useful for jumps, such as `j .`.

I am not sure who should review this. Do not hesitate to change the reviewers if needed.

Reviewers: asb, jrtc27, lenary

Reviewed By: lenary

Subscribers: MaskRay, lenary, 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/D63669

Patch by John LLVM (JohnLLVM)

llvm-svn: 365881