Headers: further refine the `stdatomic.h` MSVC inclusion

The `stdatomic.h` header cannot be included under MSVC when in C++ mode
as the toolsets prior to 17.5.0 Preview 2 do not support the C11
atomics.  However, some toolsets prior to the aforementioned version
support the atomics under the C++ mode.  In these versions the MSVC
toolset protects against the contamination by entirely eliding the
declarations, resulting in build failures in some cases.  Unfortunately,
e0c3142a accidentally regressed the C
case when fixing the C++ support.

Unfortunately, we cannot test this reliably as we need to inject a
header after the resource path as a system header path, which we do not
have a reliable means of doing.

Fixes: #59640