mplayer: prevent selection on Microblaze and AArch64

MPlayer wants to have some "support" for each architecture, and for
now, it doesn't know about Microblaze and AArch64, so prevent the
selection of MPlayer on those architectures.

For Microblaze, fixes:

  http://autobuild.buildroot.org/results/4e6e0d15694345ded649c92251cc2173dc45fe7c/build-end.log



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>