The binaries are retrieved from MSys2 on windows and built from source
on Linux.  Updates to these binaries should be infrequent enough to
not warrant a completely automated system.

Bug: angleproject:3419
Change-Id: I282f9ca14059d1fe0d88f2ffd31d02df0ea6b88f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1940571Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>