AArch64: Fix memcmp16() test

__memcmp16() should return an integer less than, equal to, or greater than
zero. However the tests looks for a specific value.

Change-Id: I06052f58f9ccc67146a3df9abb349c4bc19f090e
Signed-off-by: Serban Constantinescu <serban.constantinescu@arm.com>
diff --git a/tests/string_test.cpp b/tests/string_test.cpp
index c62f43b..67b3860 100644
--- a/tests/string_test.cpp
+++ b/tests/string_test.cpp
@@ -812,7 +812,7 @@
       int expected = (static_cast<unsigned short>(c1) - static_cast<unsigned short>(c2));
       int actual = __memcmp16(state.ptr1, state.ptr2, (size_t) state.MAX_LEN);
 
-      ASSERT_EQ(expected, actual);
+      ASSERT_EQ(signum(expected), signum(actual));
     }
   }
 #else // __BIONIC__