mpd: fix build on SPARC

mpd uses __atomic_*() built-ins, but forgets to link with -latomic
when needed, so this commit adds a patch that ensures it will link
with libatomic, and adds a dependency on BR2_TOOLCHAIN_HAS_ATOMIC.

Fixes:

  http://autobuild.buildroot.net/results/e2207280f3a177e12dba502b1c01f5aeb431d04c/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>