Add memory order constraints to C11 atomics

Reduce the strength of the C11 atomics from memory_order_seq_cst to the
actually required memory order constraints for each operation. In
addition to reflecting the already documented memory order constraints,
this should allow for better code generation.
11 jobs for c11_atomic_memory_order in 8 minutes and 28 seconds (queued for 1 second)
latest
Status Name Job ID Coverage
  Build
passed meson android arm64 fedora #13250925

00:01:21

passed meson macos #13250923
gst-macos-10.15

00:03:23

passed meson-mingw-w64-build #13250920

00:03:36

manual meson msys2 #13250924
docker windows 1809 allowed to fail manual
passed meson-shared-build #13250918

00:03:24

passed meson-static-build #13250919

00:03:46

passed meson vs2017 amd64 #13250921
docker windows 1809

00:01:16

passed meson vs2017 x86 #13250922
docker windows 1809

00:01:16

passed mingw-build #13250917

00:06:07

passed shared-build #13250915

00:08:28

passed static-build #13250916

00:06:22