make arm-use-movt and arm-reserve-r9 options available for all arm

Before this patch these flags are Darwin specific, but they are also
usefull for bare project like bootloaders / kernels etc, so make them
more commonly available.

Since arm-darwin-use-movt is only used internally it is renamed to the
more common arm-use-movt and should not cause much trouble. Even if
the arm-darwin-use-movt flag is passed llvm suggest to use the new
arm-use-movt. These options are still hidden.