[libcxx] [test] Fix max_size.pass.cpp for other Windows architectures

[mstorsjo]

Summary

All current Windows architectures (i386, x86_64, arm, arm64) get
the full_size() behaviour here. x86_64 (the only one tested in CI
currently) is handled by the first ifdef at the top, but handle
Windows in general on all other architectures later.

[philnik]

LGTM assuming CI passes.

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

 # elif defined(__arm__) || defined(__aarch64__)
 #   ifdef __BIG_ENDIAN__
   half_size();
 #   else
   full_size();
 #   endif
 # elif defined(__powerpc__) || defined(__powerpc64__)
   half_size();
+# elif defined(_WIN32)
+  full_size();
 # else
 #   error "Your target system seems to be unsupported."
 # endif
 
 #else
 
   half_size();