XCOFF constify

There are occasions where it is reasonable to use a macro defining
function parameters, but this isn't one of them.  Use typedefs
instead, which also simplifies declaring a const array of function
pointers.

	* libxcoff.h (struct xcoff_backend_data_rec): Constify
	_xcoff_glink_code.
	(XCOFF_RELOC_FUNCTION_ARGS, XCOFF_COMPLAIN_FUNCTION_ARGS): Delete.
	(xcoff_reloc_function, xcoff_complain_function): New typedef.
	(xcoff_calculate_relocation, xcoff_complain_overflow),
	(xcoff_reloc_type_noop, xcoff_reloc_type_fail),
	(xcoff_reloc_type_pos, xcoff_reloc_type_neg),
	(xcoff_reloc_type_rel, xcoff_reloc_type_toc),
	(xcoff_reloc_type_ba, xcoff_reloc_type_crel): Update declaration.
	* coff-rs6000.c (xcoff_reloc_type_br): Declare using typedef.
	(xcoff_complain_overflow_dont_func): Likewise.
	(xcoff_complain_overflow_bitfield_func): Likewise.
	(xcoff_complain_overflow_signed_func): Likewise.
	(xcoff_complain_overflow_unsigned_func): Likewise.
	(xcoff_calculate_relocation, xcoff_complain_overflow): Constify.
	(xcoff_glink_code): Constify.
	* coff64-rs6000.c (xcoff64_reloc_type_br): Declare using typedef.
	(xcoff64_calculate_relocation, xcoff64_glink_code): Constify.