This is an alternative to D85802 which added a flag for selecting between C++ ABIs at compile-time. This instead allows for overriding the platform ABI on a Fuchsia system, rather than overriding the C++ ABI.
This will be used as a way to integrate code built with Clang with code built by other compilers that don't contain Fuchsia-specific ABI changes. The immediate use case is for generating multilibs that use the generic Itanium ABI to be used by code built by GCC.