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Several of the toolchain support files in //build weren't
python3-compatible. This CL fixes those issues, at least for
the nothing-fails code path.

Also, the python2_action() wrapper script didn't handle the
case where the command line to pass to the python script
was greater than 8k limit (to cmd.exe) on windows we hit
by routing through the python.bat wrapper in depot_tools.
This CL changes how the python2_action wrapper works, to
mimic the way GN works by using python.bat to find
the path to the underlying python executable.

Bug: 1112471
Change-Id: Id628f0c46213f00f19ef2e7f58774448fe59f31d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2512387
Commit-Queue: Dirk Pranke <dpranke@google.com>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822972}