package/libbsd: further reduce the set of supported archs

Support for a,out on ARM will be dropped in Linux 3.11 (and is already
missing in the -rc).

m68k is marked BROKEN in Buildroot, so it is never ever tested.

The x86 familly is the only one left with support for a.out, now.

Reported-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>