Using ARMTargetParser in Clang

This is a starting point for using the TargetParser in Clang, in a simple
enough part of the code that can be used without disrupting the crazy
platform support that we need to be compatible with other toolchains.

Also adding a few FIXME on obvious places that need replacing, but those
cases will indeed break a few of the platform assumptions, as arch/cpu names
change multiple times in the driver.

Finally, I'm changing the "neon-vfpv3" behaviour to match standard NEON, since
-mfpu=neon implies vfpv3 by default in both Clang and LLVM. That option
string is still supported as an alias to "neon".

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@236901 91177308-0d34-0410-b5e6-96231b3b80d8