wii: workaround broken newlib

After updating devkitppc from r26 to r27, we have gcc 4.8.2 instead of 4.6.3 and newlib 2.0.0 instead of 1.2.0.
There should be proper c++11 support now and the __cplusplus version changed from `1L` to `201103L`, however there is no vsnprintf().
This should fix the wii build for now.