First cut at OS detection, taking advantage of the new generic targets.

This approach allows adding OS-specific targets/defines/etc. without 
completely breaking unknown subtargets.  No new subtargets yet, although 
I plan to add x86-Linux soon.  Others can add targets that they use as 
needed; adding a new subtarget takes very little code.

Also does some fixups for description strings; a lot of them were 
unspecified.  I think all the ones I added are correct, but 
they're unverified; corrections are welcome.

llvm-svn: 55091