set DYNAMIC_GHC_PROGRAMS to NO if platform does not support shared libs
add arm-unknown-linux to platforms which do not support shared libs

This patch disables shared libs support on arm-unknown-linux platform. It
unbreaks ghc-stage2 on this platform after recent Ian's changes
in dynamic/shared libs domain. The reason why ghc-stage2 fails when linked
with shared libs is still unknown so this is just a workaround at the moment,
but it at least recovers previous "correct" behavior of ghc-stage2
on ARM/Linux