[RISCV] Fix parseBareSymbol to not double-parse top-level operators

By failing to lex the token we end up both parsing it as a binary
operator ourselves and parsing it as a unary operator when calling
parseExpression on the RHS. For plus this is harmless but for minus this
parses "foo - 4" as "foo - -4", effectively treating a top-level minus
as a plus.

Fixes https://github.com/llvm/llvm-project/issues/54105

Reviewed By: asb, MaskRay

Differential Revision: https://reviews.llvm.org/D120635