dev-cpp/abseil-cpp: Pull in package from upstream

Pull in dev-cpp/abseil-cpp package from upstream as it will be needed
as dependency for net-libs/grpc.  Modified ebuild to add Python 3.6 to
PYTHON_COMPAT and block the dev-cpp/absl package from
chromiumos-overlay.

BUG=b:190216858
TEST=pass CQ

Change-Id: I0d23644b076099a2cdc5c8d678601f4f6bd4243b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/portage-stable/+/3383907
Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>
Commit-Queue: Sloan Johnson <sloanjohnson@google.com>
Tested-by: Sloan Johnson <sloanjohnson@google.com>
6 files changed