gas: dw2gencfi: expose a new cfi_set_last_fde API

gas/
	* dw2gencfi.c (cfi_set_last_fde): New definition.
	(dot_cfi_endproc): Use it.
	(dot_cfi_fde_data): Likewise.
	(dot_cfi_inline_lsda): Likewise.
	* dw2gencfi.h (struct fde_entry): New declaration.
	(cfi_set_last_fde): Likewise.