bpf: define __bpf__ as well as __BPF__ as a target macro

LLVM defines both __bpf__ and __BPF_ as target macros.
GCC was defining only __BPF__.

This patch defines __bpf__ as a target macro for BPF.
Tested in bpf-unknown-none.

gcc/ChangeLog:

	* config/bpf/bpf.cc (bpf_target_macros): Define __bpf__ as a
	target macro.