amd64: Pass DEBUG when building VDSO wrapper

Not doing so results in ctfconvert failing with

ERROR: ctfconvert: elf-vdso.so.o doesn't have type data to convert

On FreeBSD this is non-fatal, since the ctf tools have a hack to make
such errors not fail the build and instead just silently continue
without CTF data (which is a bad idea these days and should probably be
removed; they date back to the original import). However, those are
under #ifdef FreeBSD so do not apply when cross-building from
non-FreeBSD, causing the build to fail.

Fix this by forwarding DEBUG on to the compiler invocation for the VDSO
wrapper. It's assembly so it's not hugely useful, but there is a
non-zero amount of information preserved, and other assembly files are
built with -g by default too so this matches them; the alternative would
be to tag the files.amd64 entries with no-ctfmerge. Note that the VDSO
itself is still compiled without debug info, this only affects the
wrapper linked into the kernel.

Fixes: 98c8b6252496 ("vdso for ia32 on amd64"), ab4524b3d7fb ("amd64: wrap 64bit sigtramp into vdso")
MFC after: 1 month