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This change makes two improvements to the distributed ThinLTO scripts:
 1. When no code generation is necessary, invokes the local linker
    correctly even when --gomacc or --jobs options are specified.
 2. Change create_file to ensure_file to only create files if they
    do not already exist, reducing unnecessary rebuilding.

R=gbiv,hans,rnk,thakis
BUG=877722

Change-Id: I29ffa028202c31f77df95d306432f726e3f7bda9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2039996
Auto-Submit: Bob Haarman <inglorion@chromium.org>
Reviewed-by: George Burgess <gbiv@chromium.org>
Commit-Queue: George Burgess <gbiv@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738684}