package/grpc: needs atomic or sync built-ins

package/grpc: needs atomic or sync built-ins

grpc uses atomic or sync built-ins from the compiler, so we need to
add the appropriate dependency. In addition, on some architectures,
linking with libatomic is needed to use the atomic builtins.

Fixes:

  http://autobuild.buildroot.org/results/2f208fbfe4e9da94be5b9c030dbd278cb8ba053c/

Signed-off-by: Ryan Barnett <ryan.barnett@rockwellcollins.com>
Reviewed-by: Robert Rose <robertroyrose@gmail.com>
[Thomas:
 - add the dependency on sync or atomic builtins in Config.in
 - use -DCMAKE_EXE_LINKER_FLAGS instead of
   -DCMAKE_C_STANDARD_LIBRARIES, to be consistent with what we do in
   some other cmake-based packages
 - tweak commit log]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>