[RISCV][MC]Add support for Binary MCExpr

There is an issue: https://github.com/llvm/llvm-project/issues/64612
This issue happens because in RISCVMCCodeEmitter::getImmOpValue it only handles MCExpr kind Target and SymbolRef.
When code with format like .+ comes in, it comes with MCExpr kind Binary, the fixupkind remains fixup_riscv_invalid and reports error.

This patch make MCExpr kind Binary handled with the same way as MCExpr kind SymbolRef,
so code with binary expression can get correct fixupkind and be used to generate more complex relocation.

Differential Revision: https://reviews.llvm.org/D157694