Allow compilation without thumb-interwork as long as no thumb is

involved or only thumb2 is used.

configure.ac

@@ -871,6 +871,29 @@ if test "x$target_cpu" = xarm; then
   if test "x$grub_cv_cc_mlong_calls" = xyes; then
     TARGET_CFLAGS="$TARGET_CFLAGS -mlong-calls"
   fi
+  AC_CACHE_CHECK([whether option -mthumb-interwork works], grub_cv_cc_mthumb_interwork, [
+    CFLAGS="$TARGET_CFLAGS -mthumb-interwork -Werror"
+    AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[]], [[]])],
+		      [grub_cv_cc_mthumb_interwork=yes],
+		      [grub_cv_cc_mthumb_interwork=no])
+  ])
+  if test "x$grub_cv_cc_mthumb_interwork" = xyes; then
+    TARGET_CFLAGS="$TARGET_CFLAGS -mthumb-interwork"
+  elif test "x$grub_cv_cc_target_clang" = xno ; then
+    AC_MSG_ERROR([your compiler doesn't support -mthumb-interwork])
+  else
+    CFLAGS="$TARGET_CFLAGS"
+    AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[]], [[
+#if defined (__thumb__) && !defined (__thumb2__)
+#error thumb without interworking
+#endif
+]])],
+		      [no_interwork_ok=yes],
+		      [no_interwork_ok=no])
+    if test x$no_interwork_ok = xno ; then
+       AC_MSG_ERROR([attempt to compile to thumb with no thumb interwork])
+    fi
+  fi
 fi
 
 #