gdb/riscv: Add gdb to dwarf register number mapping

gdb/riscv: Add gdb to dwarf register number mapping

Provide a mapping between GDB's register numbers and DWARF's register
numbers.  This resolves some failures that I was seeing on
gdb.base/store.exp when running on an rv64imfdc target.

gdb/ChangeLog:

	* riscv-tdep.c (riscv_dwarf_reg_to_regnum): New function.
	(riscv_gdbarch_init): Register new function with gdbarch.
	* riscv-tdep.h: New enum to define RISC-V DWARF register numbers.