make arm-use-movt available for all ARM

Before this patch this flag is IOS specific, but is also
useful for bare project like bootloaders / kernels etc,
since movw / movt prevents simple relocation. Therefore
make this flag more commonly available.

note: this patch depends on a similiar rename in clang

Patch by Jeroen Hofstee.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188487 91177308-0d34-0410-b5e6-96231b3b80d8