Disable _Float16 for non ARM/SPIR Targets

As Discussed here:
http://lists.llvm.org/pipermail/llvm-dev/2019-January/129543.html

There are problems exposing the _Float16 type on architectures that
haven't defined the ABI/ISel for the type yet, so we're temporarily
disabling the type and making it opt-in.

Differential Revision: https://reviews.llvm.org/D57188

Change-Id: I5db7366dedf1deb9485adb8948b1deb7e612a736

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@352221 91177308-0d34-0410-b5e6-96231b3b80d8