bpf testsuite: Add BPF CO-RE tests

bpf testsuite: Add BPF CO-RE tests

This commit adds several tests for the new BPF CO-RE functionality to
the BPF target testsuite.

gcc/testsuite/ChangeLog:

	* gcc.target/bpf/core-attr-1.c: New test.
	* gcc.target/bpf/core-attr-2.c: Likewise.
	* gcc.target/bpf/core-attr-3.c: Likewise.
	* gcc.target/bpf/core-attr-4.c: Likewise
	* gcc.target/bpf/core-builtin-1.c: Likewise
	* gcc.target/bpf/core-builtin-2.c: Likewise.
	* gcc.target/bpf/core-builtin-3.c: Likewise.
	* gcc.target/bpf/core-section-1.c: Likewise.