Import the `stdatomic.h` header from FreeBSD, providing C11 atomics.
It normally comes with the compiler, not the libc (musl/glibc don't
ship it), and -nostdinc hides the compiler's copy, so anything
including it fails to build.

Pinned to a main commit, as the latest release (15.1.0) predates the
pointer atomic_fetch_{add,sub} stride fix for GCC.

Changed to:
- use compiler predefines instead of sys/cdefs.h and sys/_types.h, so it
  has no includes and one copy serves nolibc and musl. Downside:
  atomic_{u,}int_fast{16,32}_t follow the compiler's ABI, not musl's
- pick the backend w/ #ifdef __clang__; upstream's __has_extension and
  __GNUC_PREREQ__ checks always land there for our GCC-7/Clang-9 floor
- gate C23 bits on __STDC_VERSION__ (upstream's __ISO_C_VISIBLE defaults
  to 2023, i.e. on for any -std)
- make ATOMIC_VAR_INIT() (value) on GCC too: upstream's { .__val = }
  wants a struct-wrapped _Atomic and breaks with GCC's native one
- drop the pre-GCC-4.7 __sync fallback, C++ _Bool shim and _KERNEL bits

Source of import:
Repository: https://git.FreeBSD.org/src.git
Commit: 5076e9b5463afa47cf2e7689431d3dd33e5b15cb
Path: sys/sys/stdatomic.h

Checkpatch-Ignore: BRACES
Checkpatch-Ignore: CAMELCASE
Checkpatch-Ignore: COMMIT_LOG_USE_LINK
Checkpatch-Ignore: INLINE
Checkpatch-Ignore: LEADING_SPACE
Checkpatch-Ignore: MACRO_ARG_REUSE
Checkpatch-Ignore: PARENTHESIS_ALIGNMENT
Checkpatch-Ignore: SPACING
Checkpatch-Ignore: SPDX_LICENSE_TAG
Checkpatch-Ignore: VOLATILE
Signed-off-by: Sergiu Moga <sergiu@unikraft.io>