[RISCV] Attach VK_RISCV_CALL to symbols upon creation

[RISCV] Attach VK_RISCV_CALL to symbols upon creation

This patch replaces the addition of VK_RISCV_CALL in RISCVMCCodeEmitter by
creating the RISCVMCExpr when tail/call are parsed, or in the codegen case
when the callee symbols are created.

This required adding a new CallSymbol operand to allow only adding
VK_RISCV_CALL to tail/call instructions.

This patch will allow further expansion of parsing and codegen to easily
include PLT symbols which must generate the R_RISCV_CALL_PLT relocation.

Differential Revision: https://reviews.llvm.org/D55560
Patch by Lewis Revill.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@357396 91177308-0d34-0410-b5e6-96231b3b80d8