btf: fix 'extern const void' variables [PR106773]

btf: fix 'extern const void' variables [PR106773]

The eBPF loader expects to find BTF_KIND_VAR records for references to
extern const void symbols. We were mistakenly identifing these as
unsupported types, and as a result skipping emitting VAR records for
them.

In addition, the internal DWARF representation from which BTF is
produced does not generate 'const' modifier DIEs for the void type,
which meant in BTF the 'const' qualifier was dropped for 'extern const
void' variables. This patch also adds support for generating a const
void type in BTF to correct emission for these variables.

	PR target/106773

gcc/

	* btfout.cc (btf_collect_datasec): Correct size of void entries.
	(btf_dvd_emit_preprocess_cb): Do not skip emitting variables which
	refer to void types.
	(btf_init_postprocess): Create 'const void' type record if needed and
	adjust variables to refer to it as appropriate.

gcc/testsuite/

	* gcc.dg/debug/btf/btf-pr106773.c: New test.