btf: do not skip members of data type with type id BTF_VOID_TYPEID

btf: do not skip members of data type with type id BTF_VOID_TYPEID

The previous fix in gen_ctf_sou_type () exposes an issue in BTF
generation, however: BTF emission was currently decrementing the vlen
(indicating the number of members) to skip members of type CTF_K_UNKNOWN
altogether, but still emitting the BTF for the corresponding member (in
output_asm_btf_sou_fields ()).

One can see malformed BTF by executing the newly added CTF testcase
(gcc.dg/debug/ctf/ctf-bitfields-5.c) with -gbtf instead or even existing
btf-struct-2.c without this patch.

To fix the issue, it makes sense to rather _not_ skip members of data
type of type id BTF_VOID_TYPEID.

gcc/ChangeLog:
	* btfout.cc (btf_asm_type): Do not skip emitting members of
	unknown type.

gcc/testsuite/ChangeLog:
	* gcc.dg/debug/btf/btf-bitfields-4.c: Update the vlen check.
	* gcc.dg/debug/btf/btf-struct-2.c: Check that member named 'f'
	with void data type is emitted.