Compress externals using 7-zip instead of ps Compress-Archive

When I build python on my Windows laptop, the archive has 5437 files
in it and is about ~90 MB. Powershell takes 124 seconds to create the
archive, while 7-Zip takes only 5 seconds. (The Kokoro build has a
smaller archive at ~60MB because it reuses a Python install to
bootstrap whereas my local build downloads Python via nuget.)

Also: delete the output directory before running the build. It doesn't
accomplish much on Windows because PCbuild\build.bat does everything in
the source directory, but at least this rmdir will remove existing zip
files.

AFAIK the Cygwin 7z.exe is still OK, but it seems better to prefer
properly native tools, if only for faster I/O.

Bug: none
Test: kokoro presubmit, check log to see which 7z.exe was used
Change-Id: Ic8a78e200c7a951b5fb431f5742e6e648377dc8c
2 files changed