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

Use of stdatomic.h is undefined in C++, even the C++ 2020 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 provided features.

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
Reviewed by: dim, emaste
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D34686