Add support for C11 stdatomic atomics

This fixes deprecation warnings when building for macOS >= 10.12 systems.
Additionally, using stdatomic.h (or the more modern __atomic_ builtins)
is required when targeting CHERI-enabled architectures such as
CHERI-RISC-V or Arm's Morello since the compiler rejects __sync_* atomic
for pointer types (they only work with integers).
11 jobs for fix-deprecation-macos-stdatomic in 6 minutes and 21 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Build
passed meson android arm64 fedora #11966541

00:01:11

passed meson macos #11966539
gst-macos-10.15

00:03:07

passed meson-mingw-w64-build #11966536

00:04:54

manual meson msys2 #11966540
docker windows 1809 allowed to fail manual
passed meson-shared-build #11966534

00:05:04

passed meson-static-build #11966535

00:03:35

passed meson vs2017 amd64 #11966537
docker windows 1809

00:01:14

passed meson vs2017 x86 #11966538
docker windows 1809

00:01:13

passed mingw-build #11966533

00:04:42

passed shared-build #11966531

00:06:21

passed static-build #11966532

00:05:28