bpf: BPF CO-RE support

bpf: BPF CO-RE support

This commit introduces support for BPF Compile Once - Run
Everywhere (CO-RE) in GCC.

gcc/ChangeLog:

	* config/bpf/bpf.c: Adjust includes.
	(bpf_handle_preserve_access_index_attribute): New function.
	(bpf_attribute_table): Use it here.
	(bpf_builtins): Add BPF_BUILTIN_PRESERVE_ACCESS_INDEX.
	(bpf_option_override): Handle "-mco-re" option.
	(bpf_asm_init_sections): New.
	(TARGET_ASM_INIT_SECTIONS): Redefine.
	(bpf_file_end): New.
	(TARGET_ASM_FILE_END): Redefine.
	(bpf_init_builtins): Add "__builtin_preserve_access_index".
	(bpf_core_compute, bpf_core_get_index): New.
	(is_attr_preserve_access): New.
	(bpf_expand_builtin): Handle new builtins.
	(bpf_core_newdecl, bpf_core_is_maybe_aggregate_access): New.
	(bpf_core_walk): New.
	(bpf_resolve_overloaded_builtin): New.
	(TARGET_RESOLVE_OVERLOADED_BUILTIN): Redefine.
	(handle_attr): New.
	(pass_bpf_core_attr): New RTL pass.
	* config/bpf/bpf-passes.def: New file.
	* config/bpf/bpf-protos.h (make_pass_bpf_core_attr): New.
	* config/bpf/coreout.c: New file.
	* config/bpf/coreout.h: Likewise.
	* config/bpf/t-bpf (TM_H): Add $(srcdir)/config/bpf/coreout.h.
	(coreout.o): New rule.
	(PASSES_EXTRA): Add $(srcdir)/config/bpf/bpf-passes.def.
	* config.gcc (bpf): Add coreout.h to extra_headers.
	Add coreout.o to extra_objs.
	Add $(srcdir)/config/bpf/coreout.c to target_gtfiles.