binutils: Make DWARF register name lookup be via a function pointer

binutils: Make DWARF register name lookup be via a function pointer

Add a layer of indirection for DWARF register name lookup by
introducing a function pointer.  Right now all targets use the same
table based implementation that they always have, however, this will
change in a later commit.

binutils/ChangeLog:

	* dwarf.c (typedef dwarf_regname_lookup_ftype): New typedef.
	(dwarf_regnames_lookup_func): New static global.
	(init_dwarf_regnames_i386): Set dwarf_regnames_lookup_func.
	(init_dwarf_regnames_iamcu): Likewise.
	(init_dwarf_regnames_x86_64): Likewise.
	(init_dwarf_regnames_aarch64): Likewise.
	(init_dwarf_regnames_s390): Likewise.
	(init_dwarf_regnames_riscv): Likewise.
	(init_dwarf_regnames_by_elf_machine_code): Reset
	dwarf_regnames_lookup_func to NULL.
	(init_dwarf_regnames_by_bfd_arch_and_mach): Likewise.
	(regname_internal_by_table_only): New function.
	(regname): Make use of dwarf_regnames_lookup_func.

Change-Id: Ibbff61c0abea32927f35c9e478793039ab8bb57d