Remove $FreeBSD$: two-line .h pattern

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

tools/build: Don't redefine open() for the linux bootstrap

This is needed to bootstrap llvm-tblgen on Linux since LLVM calls
`::open(...)` which does not work if open is a statement macro.
Also stop defining O_SHLOCK/O_EXLOCK and update the only bootstrap tools
user of those flags to deal with missing definitions.

Reviewed By: jrtc27
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D31226

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