[RISCV] Add support for lowering floating point inlineasm clobbers

This adds the required extension to RISC-V's getRegForInlineAsmConstraint
in order to be able to correctly distringuish between the 32 and 64-bit
floating point registers when the generic fX name appears in inlineasm
clobber contraints. It also adds a check to validate that callee saved
floating point registers are only saved in this case when a hard-float
ABI is selected.

Differential Revision: https://reviews.llvm.org/D64751

llvm-svn: 367397