[RISCV] Support assembling @plt symbol operands

[RISCV] Support assembling @plt symbol operands

This patch allows symbols appended with @plt to parse and assemble with the
R_RISCV_CALL_PLT relocation.

Differential Revision: https://reviews.llvm.org/D55335
Patch by Lewis Revill.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@357470 91177308-0d34-0410-b5e6-96231b3b80d8