To (minimally) support packaging policy requirements in advance of CXX-804, this PR introduces new build configuration options which may be used by packaging to completely exclude unstable ABI symbols when building shared libraries. This additionally requires using v_noabi-specific export macros to ensure any downstream code which observes unstable ABI headers (these changes do not exclude them from installation; this will be handled by the package definition). These options are for internal use only: that is, specifically for stable ABI packaging purposes.