xen/arm: Correctly define size_t

Flask code use %zu to print size_t variable. On ARM, size_t always defined
to unsigned long which make ARM build failed.

Fix it by using __SIZE_TYPE__.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

xen/include/asm-arm/types.h

@@ -55,7 +55,11 @@ typedef u64 register_t;
 #define PRIregister "lx"
 #endif
 
+#if defined(__SIZE_TYPE__)
+typedef __SIZE_TYPE__ size_t;
+#else
 typedef unsigned long size_t;
+#endif
 
 typedef char bool_t;
 #define test_and_set_bool(b)   xchg(&(b), 1)