cross-build: Define __*int*_t and include sys/cdefs.h

FreeBSD assumes that sys/types.h includes sys/cdefs.h, so add it here.
FreeBSD also needs __*int*_t defined for software we bootstrap (a lot of
it). GLIBC defines these, but musl does not, so we have to define them
here, even though it looks backwards. There's no good #define to key off
of, so use !defined GLIBC since on Linux defacto there's only two libc
implementations.

Co-authored-by: Val Packett <val@packett.cool>
Sponsored by: Netflix
Pull Request: https://github.com/freebsd/freebsd-src/pull/1066
Reviewed by: val_packett.cool
Differential Revision: https://reviews.freebsd.org/D45354

cross-tools: Document a rather bizarre looking construct

On Linux, stddef.h will define __size_t to something when size_t is
defined. On FreeBSD, __size_t is an actual typedef which we need for
other parts of the system. If __size_t isn't defined, that tells us we
need to define the fallback __size_t typedef (if it is defined, then we
can't easily redefine it without issues because glob.h will define it to
something different than stddef.h defines it). Add a comment to this
effect.

Sponsored by: Netflix
Reviewed by: val_packett.cool
Differential Revision: https://reviews.freebsd.org/D45350

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