gold work on mingw/mingw64 support

[Ian Lance Taylor]

Vladimir Simonov <sv@sw.ru> writes:

> IMO mremap.c, pread.c, etc. are a kind of such library.
> Analysis of target OS capabilities is overkill here, we
> know in advance - mingw doesn't have mmap and most probably
> won't have it.

Sure, but there are other systems that also don't have mmap.  We don't
want to test for all of them.

> If __MINGW32__ looks too OS specific it may be
> replaced with HAVE_NO_MMAP, HAVE_NO_CLOEXEC defined
> in config/mh-mingw. Really __MINGW32__ looks quite strange
> in the middle of descriptors.cc but I don't know how to do it
> better - fake function in mremap.h would be also strange,
> new fcntl.c - the same...

I'm complaining in part about the __MINGW32__ and in part about the
#ifdef.  Apart from ENABLE_THREADS, the only #ifdefs in gold are very
small ones at the top of a few .cc files.  Let's keep it that way.

Ian