BSD 4.3 still used the Portable C Compiler. By 4.4 Berkeley was starting to use GCC, but that wasn't available in time to incorporate into A/UX. Plus, IIRC the GNU project was actively boycotting Apple at the time, and so Apple was probably adverse to incorporating any GNU software.
Greenhills was available as a third party compiler. Not sure why there would be markers in crt0 and libc, but perhaps someone at Apple rebuilt. Greenhills was better for most code that mattered.
Can't see crt0 mattering for performance, but maybe there was some interoperability glue to make both runtimes happy.
without the full source it's impossible to say. That said there was a 'tar ball' of 0.7 floating around for well over a decade, but it's able to run under emulation now.
there is a /usr/lib/greenhillls with a ccom68 fcom68 and pcom68 along with some libs and crt files. It's all binaries but I did a compare on crt0 and they match the system one. Maybe it's just a case of the crt0 being assembly and it being.. well the same assembler.
Maybe they were just investigating PCC vs Greenhills or it was a cross thing, or they built PCC with Greenhills. who knows?!
