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