tools/build: Provide sys/linker_set.h when cross-building

This is needed for kldxref, which will shortly become a bootstrap tool.
Linux can use the same one as FreeBSD (provided the cross-building
sys/cdefs.h is augmented appropriately), whilst macOS needs its own
Mach-O-specific implementation.

Reviewed by: brooks
Differential Revision: https://reviews.freebsd.org/D43049

Remove $FreeBSD$: two-line .h pattern

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

cross-build: Add __weak_symbol definition for libdwarf bootstrap

Fixes: 3aa0bc89c6a1 ("libdwarf: Add a weak uncompress() symbol")
MFC after: 1 week

Fix cross-builds after 4e5d32a445f90d37966cd6de571978551654e3f3

Add alignment macros to cross-build's sys/cdefs.h

Pull Request: https://github.com/freebsd/freebsd-src/pull/531
MFC after: immediately (build fix)

Fix warnings during bootstrap on Linux systems

Most warnings are currently off for the boostrap phase, but once D27598
lands they will be enabled again.

Fix more -Wundef warnings during bootstrap

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