binutils: Add a new function to initialise DWARF register name state

binutils: Add a new function to initialise DWARF register name state

Adds a new API function init_dwarf_regnames_by_bfd_arch_and_mach to
initialise the register name state from a BFD architecture and machine
type.

There should be no user visible changes after this commit.

binutils/ChangeLog:

	* dwarf.c (init_dwarf_regnames_by_bfd_arch_and_mach): New
	function.
	* dwarf.h (init_dwarf_regnames_by_bfd_arch_and_mach): Declare.
	* objdump.c (dump_dwarf): Call new function instead of calling
	specific initialization routines.  Restrucure so that eh_addr_size
	is still calculated correctly.

Change-Id: I346d665d2079a18ec4d04bd41893d0e9dc05e4b3