The linker script assumes that std::mbstate_t has the name __mbstate_t
for linkage purposes, but that's not necessarily true. For mingw32
it's just a typedef for int, so the patterns don't match.

This adds a new mingw32-specific pattern for codecvt_byname's
constructors and destructors, and relaxes the patterns for
codecvt<charNN_t, char, mbstate_t> so they match __mbstate_t or int.

Tested x86_64-linux and powerpc64le-linux, committed to trunk.

I plan to commit this to trunk and gcc-5-branch soon.