Support multiple .eh_frame sections

Support multiple .eh_frame sections

	This patch is based on MULTIPLE_FRAME_SECTIONS and EH_FRAME_LINKONCE,
	it allows backend to enable this feature and use '--gc-sections' simply.

	* gas/dw2gencfi.h (TARGET_MULTIPLE_EH_FRAME_SECTIONS): New.
	(MULTIPLE_FRAME_SECTIONS): Add TARGET_MULTIPLE_EH_FRAME_SECTIONS.
	* gas/dw2gencfi.c (EH_FRAME_LINKONCE): Add TARGET_MULTIPLE_EH_FRAME_SECTIONS.
	(is_now_linkonce_segment): Likewise.
	(get_cfi_seg): Create relocation info between .eh_frame.* and .text.* section.

	* bfd/elf-bfd.h (elf_backend_can_make_multiple_eh_frame): New.
	* bfd/elfxx-target.h (elf_backend_can_make_multiple_eh_frame): Likewise.
	* bfd/elflink.c (_bfd_elf_default_action_discarded): Add checking for
	elf_backend_can_make_multiple_eh_frame.