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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@55091 91177308-0d34-0410-b5e6-96231b3b80d8