patch 9.1.0502: MS-Windows: too much legacy code

patch 9.1.0502: MS-Windows: too much legacy code

Problem:  MS-Windows: too much legacy code
Solution: Clean up old code
          (Ken Takata)

* Remove very old codes for Cygwin version of GCC.
  Nowadays Cygwin GCC cannot be used for building Win32 Vim.
  (The `-mno-cygwin` option was removed in Cygwin GCC4.)
* Remove old codes for old versions of MinGW.
  Remove `__MINGW32__` as much as possible.
* Adjust makefile.

closes: #15044

Signed-off-by: K.Takata <kentkt@csc.jp>
Signed-off-by: Christian Brabandt <cb@256bit.org>