Symlink prebuilt binaries on host.

This means binaries will run from their source location and look up
shared libs relative to there.

Test: m nothing
Test: Set up a prebuilt binary with shared libs, check that it can
  be executed during build, and check that its symlinks get updated
  when a shared lib is touched.
Bug: 145934348
Change-Id: I1a600c7163ce0ec34ee8caf0ffe87fef4feb3064