Update MS Windows FAQ for MinGW64-w64/MSYS2

Update MS Windows FAQ for MinGW64-w64/MSYS2

* doc/misc/efaq-w32.texi (Compiling, Debugging): Mention
MinGW64-w64/MSYS2 as the preferred way for building Emacs on
capable systems.
(Attachments with Gnus): Catch up with emacs-mime.texi in the
example given.
(Spell check): Add the availability of GNU Aspell and Hunspell
in MSYS2 distribution.
(Other useful ports): Add an entry for MinGW64-w64/MSYS2.
Fix link for MinGW homepage.
Remove entry for defunct UWIN project.  (Bug#55930)