[libc++][test] Fix max_size test for Arm big endian

[michaelplatings]

Summary

With ABI version 2 and big endian Arm, the full_size test matches the
current behaviour. It's possible that this would also be the case for
other big endian architectures but I don't have data on that.

libcxx/test/libcxx/strings/basic.string/string.capacity/max_size.pass.cpp

 # elif defined(_WIN32)
   full_size();
 # else
 #   error "Your target system seems to be unsupported."
 # endif
 
 #else
 
+# if defined(__arm__) || defined(__aarch64__)
+#   ifdef __BIG_ENDIAN__
+  full_size();
+#   else
   half_size();
+#   endif
+# else
+  half_size();
+# endif
 
 #endif
 
   return true;
 }
 
 int main(int, char**) {
   test();
 #if TEST_STD_VER > 17
   static_assert(test());
 #endif
 
   return 0;
 }