Allow bootstrapping pwd_mkdb on Linux/macOS

We need to provide a struct passwd that is compatible with the target
system and this is not the case when cross-building from macOS/Linux.
It should also be a problem when bootstrapping for an i386 target from a
FreeBSD amd64 host since time_t does not match across those systems.
However, pwd_mkdb always truncates integer values to 32-bit so this
difference does not result in different databases.

Reviewed By: brooks
Differential Revision: https://reviews.freebsd.org/D25931