patch 9.1.0621: MS-Windows: startup code can be improved

patch 9.1.0621: MS-Windows: startup code can be improved

Problem:  MS-Windows: startup code can be improved
Solution: Re-work and optimize win32 startup code
          (Ken Takata)

* Revise the code and reduce #ifdefs.
* For VIMDLL, stop using the default CRT startup code to reduce the file
  size.
  The file size becomes ~130 KB -> ~34 KB on MSVC.
* Update comments. Make them consistent between os_w32dll.c and
  os_w32exe.c.

closes: #15352

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