Add support for using GCC/Clang __atomic_* builtins

These builtins are the more recent replacement for __sync_* atomic builtins.
Use of these builtins is required when targetting CHERI-enabled
architectures such as CHERI-RISC-V or Arm's Morello since the compiler
does not support __sync_* atomic for pointer types (they only work
with integers).
11 jobs for gcc-atomic-builtins in 10 minutes (queued for 4 seconds)
latest
Status Job ID Name Coverage
  Build
passed meson android arm64 fedora #11710870

00:01:28

passed meson macos #11710868
gst-macos-10.15

00:05:35

passed meson-mingw-w64-build #11710865

00:07:54

manual meson msys2 #11710869
docker windows 1809 allowed to fail manual
passed meson-shared-build #11710863

00:08:41

passed meson-static-build #11710864

00:06:09

passed meson vs2017 amd64 #11710866
docker windows 1809

00:03:10

passed meson vs2017 x86 #11710867
docker windows 1809

00:03:09

passed mingw-build #11710862

00:07:30

passed shared-build #11710860

00:09:10

passed static-build #11710861

00:09:59