make: autoconf: define ARM for arm build with clang

Fixes error like

ad_arm.cpp:19849:4: error: "ARM must be defined"
   ^
1 warning and 1 error generated.
gmake[3]: *** [lib/CompileJvm.gmk:143: /home/builder/.termux-build/openjdk-17/src/build/linux-arm-server-release/hotspot/variant-server/libjvm/objs/ad_arm.o] Error 1
gmake[2]: *** [make/Main.gmk:252: hotspot-server-libs] Error 2

make/autoconf/flags-cflags.m4

@@ -708,7 +708,12 @@ AC_DEFUN([FLAGS_SETUP_CFLAGS_CPU_DEP],
         $1_CFLAGS_CPU_JDK="${$1_CFLAGS_CPU_JDK} -fno-omit-frame-pointer"
       fi
     fi
-
+    if test "x$FLAGS_CPU" = xarm; then
+      # For whatever reason arm build with clang fails with:
+      #   ad_arm.cpp:19849:4: error: "ARM must be defined"
+      # Fix by defining arm here as well.
+      $1_CFLAGS_CPU_JVM="-DARM"
+    fi
   elif test "x$TOOLCHAIN_TYPE" = xxlc; then
     if test "x$FLAGS_CPU" = xppc64; then
       $1_CFLAGS_CPU_JVM="-qarch=ppc64"