Fix 'gn gen' with target_os="win" on Linux.
Fix 'gn gen' with target_os="win" on Linux.

This is the first step to enable Windows cross-compilation when the
host_os is Linux.

There are compilation errors ahead but at least it is possible to
generate ninja files.

Bug: webrtc:8875
Change-Id: I91a238bcb5e8f7670a6d19805e1ac032511fd46e
Reviewed-on: https://webrtc-review.googlesource.com/51821
Reviewed-by: Patrik Höglund <b486136ee68458b09c92c86c39f451c87d6ef6a1@webrtc.org>
Commit-Queue: Mirko Bonadei <d2c43c210eae6feef04f53bae50885e8152edcca@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#21984}
ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc