[libcxx] [test] Make the put_long_double test pass on mingw, clarify quirks in put_double

Expect the same NAN formatting on Windows as on Glibc. (Both MSVC and
MinGW produce the same formatting there.)

The hex float formatting tests pass on MinGW, so opt in to those tests.

Document exactly what issues are remaining in Clang-cl/MSVC
configurations. (It's easily possible to make the tests pass there too,
but it requires a whole lot of small-scope ifndefs in the test file;
around 60 ifdefs in total for those both test files. Those could
be avoided if the CI environment could run with a newer version
of UCRT, but that's nontrivial to fix right away.)

Differential Revision: https://reviews.llvm.org/D119766