Remove $FreeBSD$: two-line .h pattern

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

Allow bootstrapping llvm-tblgen on macOS and Linux

This is needed in order to build various LLVM binutils (e.g. addr2line)
as well as clang/lld/lldb.

Co-authored-by: Jessica Clarke <jrtc27@FreeBSD.org>
Test Plan: Compiles on ubuntu 18.04 and macOS 11.4
Reviewed By: dim
Differential Revision: https://reviews.freebsd.org/D31057

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