sys/stdatomic.h: be nicer to c++

[kib]

Summary

Use of stdatomic.h is undefined in C++, even C++ 2000 standard does not
list stdatomic.h as a C library header supported by the language.  More,
there are some subtle differences between the <atomic> C++ header, and
C11+ stdatomic.h.

Nonetheless, it is a quality of the implementation aspect, so let mis-users
mis-use stdatomic.h as they want, by making a compat shim for _Bool.

PR:     262683
Reported by:    yuri

[dim]

LGTM. Have you seen a particular piece of C++ software that either accidentally or purposefully includes stdatomic.h?

[kib]

In D34686#785864, @dim wrote:

LGTM. Have you seen a particular piece of C++ software that either accidentally or purposefully includes stdatomic.h?

I reacted to the PR, nothing more. Perhaphs yuri@ can comment more.

[yuri]

In D34686#785864, @dim wrote:

LGTM. Have you seen a particular piece of C++ software that either accidentally or purposefully includes stdatomic.h?

JUCE framework includes <stdatomic.h> here: https://github.com/surge-synthesizer/JUCE/blob/086b21be1b08442631dcb58a9abc12acd48fce0a/modules/juce_audio_processors/format_types/VST3_SDK/pluginterfaces/base/funknown.cpp#L59