rfc and [patch]: Unify -mfpu and .fpu handling, let .fpu toggle features

[Rafael Avila de Espindola]

> One drawback is that the new ARMSubtargetCommon.cpp file is in the library ARMCodeGen, so clang's driver now depends on ARMCodeGen, which is a bit icky. I could make it a standalone target I suppose, but that standalone target's cpp file depends on the ARMCodeGen target's tablegen output, so that wouldn't be a huge win. And since the driver processes flags related to ARMCodeGen, the dependency doesn't seem _that_ icky to me.
> 
> (There are several places in the driver that say "FIXME: this is duplicating subtarget code", both for ARM and x86, this patch removes one of them.)

Does the driver need to process the  -mfpu just to pass the results to -cc1 or is the information needed for something like deciding which libraries to link with? If just -cc1, maybe we could just pass down the raw option? Would this help with the dependency? The front end at least would still have the dependency to find out which defines to produce I think.

Cheers,
Rafael