dev-util/spirv-tools: Uprev to 2020.6

Required by uprev of vulkan-layers.

This is a copy of the upstream ebuild with the following change:
  - Add python_36 to PYTHON_COMPAT.

BUG=b:195986385
TEST=emerge-$BOARD spirv-tools

Change-Id: Ie5aacf9cd851e8f81a454207469191ae01a27974
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/portage-stable/+/3098399
Tested-by: Chad Versace <chadversary@chromium.org>
Auto-Submit: Chad Versace <chadversary@chromium.org>
Reviewed-by: Chia-I Wu <olv@google.com>
Reviewed-by: Matt Turner <msturner@google.com>
Commit-Queue: Chad Versace <chadversary@chromium.org>
3 files changed