binutils/riscv: Register names in DWARF output

Adds a register name table for RiscV so that objdump and readelf can
both use better register names.

binutils/ChangeLog:

	* dwarf.c (dwarf_regnames_riscv): New register name table.
	(init_dwarf_regnames_riscv): New function.
	(init_dwarf_regnames): Add call to initialise RiscV register
	names.
	* dwarf.h (init_dwarf_regnames_riscv): Declare.
	* objdump.c (dump_dwarf): Add call to initialise RiscV register
	names.