[RISCV] Avoid std::pair<> in FPReg StringSwitch to avoid MSVC compile failures. NFCI.

As discussed on D90322, some MSVC builds are failing with is_trivially_copyable static asserts (see D86126) - we can avoid this by not using the std::pair<unsigned,unsigned> which held both the FP+DP Registers, just handle the FP register and convert to DP on the fly.