Restore fcatomic compatibility with Mac OS X 10.4.

Reverts the part of 6def6616 that
removed the fc_atomic_ptr_cmpexch compatibility code for systems earlier
than Mac OS X 10.5.

Reverts the part of 447b9ccc that moved
the definition of fc_atomic_ptr_get into the Mac OS X > 10.4 block. That
code is still needed on 10.4 and is implemented using functions that are
available on 10.4.

Compare against MAC_OS_X_VERSION_MIN_REQUIRED using a number not a
constant because the constant is not available on earlier SDKs.
11 jobs for ryandesign-fcatomic-tiger in 9 minutes and 58 seconds (queued for 4 seconds)
Status Name Job ID Coverage
  Build
passed meson android arm64 fedora #11927551

00:02:06

passed meson macos #11927549
gst-macos-10.15

00:02:47

passed meson-mingw-w64-build #11927546

00:06:36

manual meson msys2 #11927550
docker windows 1809 allowed to fail manual
passed meson-shared-build #11927544

00:06:45

passed meson-static-build #11927545

00:09:14

passed meson vs2017 amd64 #11927547
docker windows 1809

00:01:21

passed meson vs2017 x86 #11927548
docker windows 1809

00:01:21

passed mingw-build #11927543

00:09:56

passed shared-build #11927541

00:08:45

passed static-build #11927542

00:07:27