make arm-use-movt available for all ARM

This updates clang according to a pending patch for llvm to
rename of the -arm-darwin-use-movt to arm-use-movt to make
it available for all of ARM.

note: please apply this close to the llvm change.

Patch by Jeroen Hofstee.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188488 91177308-0d34-0410-b5e6-96231b3b80d8