bazel cc_toolchain: convert to AOSP specific platforms.

This changes the exec and target platforms to use the custom platforms
defined in //build/bazel/platforms/BUILD.bazel, to ensure that we
accurately mirror the support matrix of arch and OS in Android Platform.

Test: TH
Change-Id: I7cbf61dc3b4127cd4e8b4fbfbf87ab2a9999fbec