Constify elf_backend_eh_frame_address_size

Constify elf_backend_eh_frame_address_size

	* elf-bfd.h (struct elf_backend_data): Make asection param of
	elf_backend_eh_frame_address_size const.
	(_bfd_elf_eh_frame_address_size): Likewise.
	* elf32-m32c.c (_bfd_m32c_elf_eh_frame_address_size): Likewise.
	* elf32-msp430.c (elf32_msp430_eh_frame_address_size): Likewise.
	* elfxx-mips.c (_bfd_mips_elf_eh_frame_address_size): Likewise.
	* elfxx-mips.h (_bfd_mips_elf_eh_frame_address_size): Likewise.
	* elf-eh-frame.c (_bfd_elf_eh_frame_address_size): Likewise.
	(next_cie_fde_offset): Constify params.
	(offset_adjust, adjust_eh_frame_local_symbols): Likewise.