gcc.c's check_offload_target_name: Fixes to inform hints

gcc.c's check_offload_target_name: Fixes to inform hints

gcc/ChangeLog:

	* gcc.c (close_at_file, execute): Replace alloca by XALLOCAVEC.
	(check_offload_target_name): Fix splitting OFFLOAD_TARGETS into
	a candidate list; better inform no offload target is configured
	and fix hint extraction when passed target is not '\0' at [len].
	* common.opt (foffload): Add tailing '.'.
	(foffload-options): Likewise; fix flag name in the help string.