Add FASTLINK to windows builds

From my local testing, this brings incremental link times back where
they were before switching to Z7 (55s -> 15s). It also speeds up full
builds times by a similar margin (so no real impact on per-translation
unit build time).

https://blogs.msdn.microsoft.com/vcblog/2016/10/05/faster-c-build-cycle-in-vs-15-with-debugfastlink/

Bug: skia:
Change-Id: I83a3b8a3d6492eb9a6bd7fe8b6893b2dd6970b06
Reviewed-on: https://skia-review.googlesource.com/89800
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>

gn/BUILD.gn

@@ -381,7 +381,7 @@ config("debug_symbols") {
     ]
   } else if (is_win) {
     cflags = [ "/Z7" ]
-    ldflags = [ "/DEBUG" ]
+    ldflags = [ "/DEBUG:FASTLINK" ]
   } else {
     cflags = [ "-g" ]
   }