[RISCV] Implement MC layer support for the tail pseudoinstruction

Summary:
This patch implements MC support for tail psuedo instruction.
A follow-up patch implements the codegen support as well as handling of the indirect tail pseudo instruction.

Reviewers: asb, apazos

Reviewed By: asb

Subscribers: rbar, johnrusso, simoncook, jordy.potman.lists, sabuasal, niosHD, kito-cheng, shiva0217, zzheng, edward-jones, llvm-commits

Differential Revision: https://reviews.llvm.org/D46221

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@332634 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed