The reason this is required is for two reasons: 1) the libc
targeting `aarch64` macOS is slightly newer than that targeting
`x86_64`, and 2) `OSAtomic.h` uses relative imports rather than
system-wide imports for accompanying headers which clearly is an
oversight on Apple's part. Until such time when `libkern` headers
between `x86_64` and `aarch64` are identical, this will require a
manual intervention to duplicate the relevant headers between the
respective architectures.
See ziglang/fetch-them-macos-headers#2 for more details. The path
forward looks like one of the following:
* Ony provide headers for the oldest supported macOS (currently 10.13
but soon to be bumped to 10.14).
* Provide headers for multiple versions, and select based on the Zig
target OS version range minimum.
* Don't try to provide macOS headers.
If we don't tackle the version problem, we would have to re-introduce
the ability to detect and depend on native system headers if we wanted
to support C/C++ code that used newer OS definitions.
This patch also adds support for `#include <mach/mach_time.h>`.
Also related: #5236
