[RISCV][MC] Use a custom ParserMethod for the bare_symbol operand type

This allows the hard-coded shouldForceImmediate logic to be removed because 
the generated MatchOperandParserImpl makes use of the current context (i.e. 
the current mnemonic) to determine parsing behaviour, and so won't first try 
to parse a register before parsing a symbol name.

No functional change is intended. gas accepts immediate arguments for call, 
tail and lla. This patch doesn't address this discrepancy.

Differential Revision: https://reviews.llvm.org/D51733

llvm-svn: 342488