tools: moveconfig: add Xtensa GCC prefix to CROSS_COMPILE list

This is needed to move CONFIG options for the recently-added
xtfpga_defconfig.

The tarball of the pre-built toolchain can be downloaded from:
https://www.kernel.org/pub/tools/crosstool/files/bin/x86_64/4.9.0/Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

tools/moveconfig.py

@@ -203,7 +203,8 @@ CROSS_COMPILE = {
     'powerpc': 'powerpc-linux-',
     'sh': 'sh-linux-gnu-',
     'sparc': 'sparc-linux-',
-    'x86': 'i386-linux-'
+    'x86': 'i386-linux-',
+    'xtensa': 'xtensa-linux-'
 }
 
 STATE_IDLE = 0