Fix vdso compat32 timekeeping on !=x86.

Since x86 is the only 32-bit arch that has a 32-bit time_t, adjust the
private bintime32 struct in vdso to only use a 32 bit sec on amd64.

This matches the existing behavior in the compat code.

Noticed while implementing vdso timekeeping on powerpc.

This should also theoretically fix vdso timekeeping for arm binaries on
aarch64.

See tools attached to https://reviews.freebsd.org/D26347 for testing.

Reviewed by: kib (in irc)
Sponsored by: Tag1 Consulting, Inc.