bpf: avoid possible null deref in btf_ext_output [PR target/117447]

bpf: avoid possible null deref in btf_ext_output [PR target/117447]

The BPF-specific .BTF.ext section is always generated for BPF programs
if -gbtf is specified, and generating it requires BTF information and
assumes that the BTF info has already been generated.

Compiling non-C languages to BPF is not supported, nor is generating
CTF/BTF for non-C.  But, compiling another language like C++ to BPF
with -gbtf specified meant that we would try to generate the .BTF.ext
section anyway, and then ICE because no BTF information was available.

Add a check to bail out of btf_ext_output if the TU CTFC does not exist,
meaning no BTF info is available.

gcc/
	PR target/117447
	* config/bpf/btfext-out.cc (btf_ext_output): Bail if TU CTFC is null.