cross: Move Solaris API64 defines to common

off64_t is needed for both Linux (musl) and MacOS, so move them to the
common area. Somehow glibc provides the definition, but defining it
doesn't hurt and hels in the musl case.

Reviewed by: allanjude, jrtc27
Pull Request: https://github.com/freebsd/freebsd-src/pull/1066