bpf: remove huge memory waste with string allocation.

The BPF backend was allocating an unnecessarily large string when
constructing CO-RE relocations for enum types.
This patch also verifies that those enumerators are valid for CO-RE,
returning an error otherwise.

gcc/ChangeLog:
	* config/bpf/core-builtins.cc (get_index_for_enum_value): Create
	function.
	(pack_enum_value): Check for enumerator and error out.
	(process_enum_value): Correct string allocation.