MinGW-w64 always uses "w64" as vendor key which the previously
used check for "64" anywhere in the target triplet matched. This
would lead to MinGW-w64 setups always being treated as x64.

Since we are actually interested in finding the "64" in the
triplet's operating system key, this fix makes sure to look for
"mingw64" and thus avoids the false match.

Fixes issue #3835. Credits to Lode Leroy for the original patch.