Simplify windows-nat.c:windows_make_so #ifdefery

There are two separate #ifndef __CYGWIN__/#else/#endif sections in the
windows_make_so function with 3 lines of shared code separating them.
I find this makes the code harder to understand than necessary.
AFAICS, there is no reason those three shared lines need to be after
the first #ifdef block.  There is no early return, nor are 'load_addr'
nor 'name' modified.

This commit moves that shared code to the top of the function, and
then combines the two #ifndef sections.

Approved-By: John Baldwin <jhb@FreeBSD.org>
Change-Id: If2678b52836b1c3134a5e9f9fdaee74448d8b7bc