track my github clone of clang
make arm-use-movt and arm-reserve-r9 options for all arm

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