Revert "Revert "mixed builds correctly reference stubs libs""

This reverts commit 1db43487.

Changes from original:
  - extracted function to add the current version to stub
    versions for bp2build.
  - added libc++ to mixed builds denylist

Reason for revert: re-uploading with fix in topic

Change-Id: Ifa0ed456bf8cb4a7f861d6826263adfedb4fdd9c