amd64 ia32 vdso: always define some __vdso_ symbols

... regardless of the kernel config options.
It is reported that llvm16 ld.lld warns about undefined symbols
referenced by the VERSION script.

Reviewed by: emaste, val_packett.cool
Discussed with: jrtc27
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D38392