Disable movw/movt with clang

[Andrei Borzenkov]

> diff --git a/configure.ac b/configure.ac
> index 4e980c5..ab7fa92 100644
> --- a/configure.ac
> +++ b/configure.ac
> @@ -1152,6 +1152,16 @@ if test "$target_cpu"-"$platform" = x86_64-efi; then
>  fi
>  
>  if test "x$target_cpu" = xarm; then
> +  AC_CACHE_CHECK([whether option -mllvm -arm-use-movt=0 works], 
> grub_cv_cc_mllvm_arm_use_movt, [
> +    CPPFLAGS="$TARGET_CPPFLAGS -mllvm -arm-use-movt=0 -Werror"
> +    AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[]], [[]])],
> +                     [grub_cv_cc_mllvm_arm_use_movt=yes],
> +                     [grub_cv_cc_mllvm_arm_use_movt=no])
> +  ])
> +  if test "x$grub_cv_cc_mllvm_arm_use_movt" = xyes; then
> +    # A trick so that clang doesn't see it on link stage
> +    TARGET_CPPFLAGS="$TARGET_CPPFLAGS -mllvm -arm-use-movt=0"
> +  fi
>    AC_CACHE_CHECK([whether option -mlong-calls works], 
> grub_cv_cc_mlong_calls, [
>      CFLAGS="$TARGET_CFLAGS -mlong-calls -Werror"
>      AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[]], [[]])],

Is this option supported by all clang versions? What should we do if
this is *not* supported? As far as I understand commit message this
leads to broken code?

Is "-mllvm -arm-use-movt=0" different from "-mno-movt" which appears to
be "official" way to disable it?