GN does not like multiple files in the same target that share a name, so
add a script to check for this. A follow-up changes will hook that up to
the builders, so we'll flag this in try jobs rather than when the change
trickles downstream.

Change-Id: Ic413dd9aeed6da54fc85dea07f80fe7084be9e9e
Reviewed-on: https://boringssl-review.googlesource.com/28844
Reviewed-by: Adam Langley <agl@google.com>