bpf: set PREFERRED_DEBUGGING_TYPE to BTF_DEBUG

BTF is the standard debug info used with BPF programs, so it makes sense
to default to BTF rather than DWARF.

gcc/
	* config/bpf/bpf.h (PREFERRED_DEBUGGING_TYPE): Set to BTF_DEBUG.

gcc/testsuite/
	* gcc.target/bpf/bpf-debug-options-1.c: New test.
	* gcc.target/bpf/bpf-debug-options-2.c: Likewise.
	* gcc.target/bpf/bpf-debug-options-3.c: Likewise.
	* gcc.target/bpf/core-options-4.c: Likewise.