Correct coff-aarch64 howtos and delete unnecessary special functions

Correct coff-aarch64 howtos and delete unnecessary special functions

The remaining special functions are still broken except when called
by gas bfd_install_relocation.

	* coff-aarch64.c (coff_aarch64_addr64_reloc),
	(coff_aarch64_addr32_reloc, coff_aarch64_branch26_reloc),
	(coff_aarch64_branch19_reloc, coff_aarch64_branch14_reloc),
	(coff_aarch64_po12a_reloc): Delete.
	(HOWTO_INSTALL_ADDEND): Define as 1.
	(HOW): Remove pcrel_off.  Correct all the howtos.
	(CALC_ADDEND): Define.
	(coff_aarch64_rtype_to_howto): New function.
	(coff_rtype_to_howto): Define.