RISCV: Move RISCVMCExpr functions to RISCVMCAsmInfo or RISCVMCAsmBackend
* Move getPCRelHiFixup closer to the only caller RISCVAsmBackend::evaluateTargetFixup.
* Declare getSpecifierForName in RISCVMCAsmInfo, in align with other
targets that have migrated to the new relocation specifier representation.