Get generate_system_modulemap working on windows.

Note that since the allowlist has not yet been configured for windows,
it generates a valid modulemap and no longer crashes, but the system
module does not yet compile.

Bug: 500845363
Link: https://chromium-review.googlesource.com/id/Ib8dce57d0cd118332ed0ad0311947dab6a6a6964
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7844094
Commit-Queue: Matt Stark <msta@google.com>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1629774}
NOKEYCHECK=True
GitOrigin-RevId: e31ba601989c8fdf7d3307095dbeb2bd6a2311eb