libdrm: improve atomic handling, fix SPARCv8 build

libdrm: improve atomic handling, fix SPARCv8 build

This commit improves the handling of the "atomic stuff" in the libdrm
package. libdrm can either use the atomic intrinsics (4 byte variant)
when available, or otherwise can use libatomic_ops. Note that the
dependency on atomic operations is not from libdrm itself, but only
from some specific DRM drivers only.

Amongst other things, it fixes the build of the libdrm package on
SPARCv8, therefore fixing:

  http://autobuild.buildroot.org/results/74dd29b5ea146c320fde80a87a2fc910de9b7f60/

This commit does a number of changes that are all related to each
other:

 - Removes the dependency of the Intel DRM driver on
   libatomic_ops. The Intel DRM driver builds perfectly fine without
   libatomic_ops, as long as 4-byte variant __sync operations are
   available, which is always the case on x86 and x86_84 (which are
   the only architectures on which the Intel DRM driver can be
   enabled).

 - Adds an hidden Config.in boolean option
   BR2_PACKAGE_LIBDRM_HAS_ATOMIC that allows DRM driver that need
   atomic operation to know whether atomic support is available
   (either through intrinsics or through libatomic_ops).

 - Adds an hidden BR2_PACKAGE_LIBDRM_ENABLE_ATOMIC Config.in option
   that DRM drivers that need atomic operation should select to ensure
   that the relevant dependencies are selected. It simply selects
   libatomic_ops if 4-byte atomic intrinsics are not available. We
   could let each DRM driver do this, but having an intermediate
   option avoids a bit of duplication.

 - Adds a patch that defines AO_REQUIRE_CAS before including
   <atomic_ops.h>. This is needed because libdrm uses the
   AO_compare_and_swap_full() which is only provided on all
   architectures when AO_REQUIRE_CAS is defined. The exact same fix
   was done in the erlang package in commit
   4a9df2942470241d7a96f326f0e7012aacd36f2e.

 - Adds the dependency on libatomic_ops when the package is enabled,
   and passes the necessary CFLAGS on SPARCv8 to make the thing build
   properly. The same CFLAGS are passed in the nginx package and bdwgc
   package.

Cc: Waldemar Brodkorb <wbx@openadk.org>
Cc: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>