[RISCV] Add support for -ffixed-xX flags
This adds support for reserving GPRs such that the compiler will not
choose a register for register allocation. The implementation follows
the same design as for AArch64; each reserved register becomes a target
feature and used for getting the reserved registers for a given
MachineFunction. The backend checks that it does not need to write to
any reserved register; if it does a relevant error is generated.
Differential Revision: https://reviews.llvm.org/D67185