ARMv7-debian assembler fixes

This commit is needed for pocl to be able to compile kernels on ARM.
The integrated LLVM assembler is still broken (I'll send bugs to LLVM about this, but I don't think these will end up to 3.4), so a work-around the driving of GNU as is needed. 
Doe the change (remove HOST_CLANG_FLAGS from clang when acting as assembler driver) break for anyone. If so, the driving command needs to be #ifdef'ed.