binutils: Rename init_dwarf_regnames

As part of a process to change how dwarf.c figures out the correct
name for a register I wanted to clean up how we initialise the
register name tracking state.

As part of this I rename init_dwarf_regnames to
init_dwarf_regnames_by_elf_machine_code, later commits will add a
different entry point to initialise the register name state.

There should be no user visible changes after this commit.

binutils/ChangeLog:

	* dwarf.c (init_dwarf_regnames): Renamed to...
	(init_dwarf_regnames_by_elf_machine_code): ...this.
	* dwarf.h (init_dwarf_regnames): Renamed to...
	(init_dwarf_regnames_by_elf_machine_code): ...this.
	* readelf.c (process_file_header): Update call to use new name.

Change-Id: Ic8d2ef5fb62a8590ecd8cbb7e6258e11c6263594