Unify path delimiters in win32.mak

[e10s]

Currently '/' and '\' are mixedly used but this PR replaces the former with the latter to avoid possible confusion.

[WalterBright]

It's been like this for 17 years now, and there are no reports of confusion. The trouble with mass changes like this is they make things harder to track with git blame. I need a better reason to approve this.

[e10s]

Okay, I've been hacking broken win64 build system and found this inconsistency. E.g., $G/strtold.obj on win32 against win64's $D\strtold.obj and $G\longdouble.obj against $D\longdouble.obj. In addition, the MAKE program distinguishes '/' and '\'. Because these confused me, I thought they should be unified for coming hacks.

Overkill? Should be a just follow-up of the recent PR #6562?

[RazvanN7]

I agree with this PR not because the inconsistent use of \ and / is confusing, but rather annoying.
CC @WalterBright

[ibuclaw]

I agree with Walter here too, sorry. Closing as this has stalled.