Phobos' Windows Makefile

[Eduard Staniloiu]

Hello,

I've hit the following problem on this PR [0]:
The Windows 32bit build fails with the error: "more than 32767 
symbols in object file" [1].

After taking a look in `win32.mak`, I've seen that we are 
bundling multiple source files into a single object (which is the 
issue here), instead of compiling each source file into it's 
corresponding object and then linking the objects together.
Is there a reason behind doing so, or should we rewrite the 
Windows makefiles?

Looking forward to your answers! :)
Cheers,
Eduard

[0] - https://github.com/dlang/phobos/pull/5331
[1] - 
https://auto-tester.puremagic.com/show-run.ghtml?projectid=1&runid=2826646&isPull=true