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.
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.