Remove $FreeBSD$: two-line .h pattern

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

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