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

Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

Fix dtrace tools bootstrap on non-FreeBSD after OpenZFS import

This required surprisingly few build system changes and only two changes to the
openZFS compat headers which have been upstreamed as
https://github.com/openzfs/zfs/pull/10863

Reviewed By: #zfs, freqlabs
Differential Revision: https://reviews.freebsd.org/D26193

Add Linux/macOS compatibility system headers to tools/build/cross-build

These headers are required in order to build the bootstrap tools on macOS
and Linux. A follow-up commit will add implementations of functions that
don't exist on those operating systems to -legacy when bootstrapping.

Reviewed By: brooks
Differential Revision: https://reviews.freebsd.org/D14316