bpf: Use enum for resolved overloaded builtins

bpf: Use enum for resolved overloaded builtins

Change several places in the eBPF backend dealing with overloaded
built-in functions to consistently use the enum bpf_builtins type,
rather than variously using integer constants or booleans. The result is
eaiser to read and extend.

gcc/

	* config/bpf/bpf.cc (struct core_walk_data): Add field `which'...
	(bpf_resolve_overloaded_builtin): ... set it here. Use values of enum
	bpf_builtins for error checks.
	(bpf_core_walk): Use values of enum bpf_builtins.
	(bpf_core_newdecl): Likewise.
	(bpf_expand_builtin): Likewise.