To temporarily work around build issues related to Google3 integration,
make ASTC support optional and only enable it for CMake, Chromium, and
Android builds.

Bug: b/150600814
Change-Id: Iac6a404322c65a6f2727807ea8af2edfb219a5a2
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/41828Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Sean Risser <srisser@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>