GN win: prep_libc and fix tcmalloc linkage

On top of https://codereview.chromium.org/290633012/

R=brettw@chromium.org

Review URL: https://codereview.chromium.org/290713002

git-svn-id: http://src.chromium.org/svn/trunk/src/build@272249 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

config/win/visual_studio_version.gni

@@ -19,4 +19,3 @@ if (visual_studio_path == "") {
   visual_studio_path = toolchain_data[0]
   windows_sdk_path = toolchain_data[1]
 }
-