Extract SEH shared helpers into separate file

The patch moves SEH helpers to a separate shared file,
which will be reused by the SEH implementation on AArch64.

Signed-off-by: Evgeny Karpov <evgeny@kmaps.co>

gas/ChangeLog:

	* config/obj-coff-seh.c (struct seh_seg_list): Move into
	  obj-coff-seh-shared.c.
	(get_pxdata_name): Likewise.
	(alloc_pxdata_item): Likewise.
	(make_pxdata_seg): Likewise.
	(seh_hash_insert): Likewise.
	(seh_hash_find): Likewise.
	(seh_hash_find_or_make): Likewise.
	(seh_validate_seg): Likewise.
	(switch_xdata): Likewise.
	(switch_pdata): Likewise.
	(verify_context): Likewise.
	(skip_whitespace_and_comma): Likewise.
	* config/obj-coff-seh.h (OBJ_COFF_SEH_H): Add guard.
	(obj_coff_seh_code): Likewise.
	* config/obj-coff.c: Update.
	* config/obj-coff-seh-shared.c: New file.