<xatomic.h>: Investigate making this a core header

[StephanTLavavej]

<xatomic.h> is currently a non-core header:

STL/stl/inc/xatomic.h

Line 9 in ac129e5

#include <yvals.h>

In #3048, we found that we might want to centralize some macro machinery here, but before we drag it into the import lib, <xatomic.h> really ought to be a core header (as we've had trouble with non-core headers getting into the import lib).

It should be possible to make this into a core header with a bit of investigation.

[frederick-vs-ja]

I've noticed that there are several internal core headers that are never included by standard core headers. Can we add a test for their core-ness?

[CaseyCarter]

I've noticed that there are several internal core headers that are never included by standard core headers. Can we add a test for their core-ness?

std/tests/GH_001411_core_headers enforces core-ness.