bpf: Add new -mco-re option for BPF CO-RE

bpf: Add new -mco-re option for BPF CO-RE

-mco-re in the BPF backend enables code generation for the CO-RE usecase. LTO is
disabled for CO-RE compilations.

gcc/ChangeLog:

	* config/bpf/bpf.c (bpf_option_override): For BPF backend, disable LTO
	support when compiling for CO-RE.
	* config/bpf/bpf.opt: Add new command line option -mco-re.

gcc/testsuite/ChangeLog:

	* gcc.target/bpf/core-lto-1.c: New test.