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.
https://macintoshgarden.org/apps/aux-apple-unix-68k-version-...
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?!