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.

Out of the box running strings on stuff shows the 1984-1985 AT&T-IS 1985-1987 UniSoft Corporation strings.

So it's got to be PCC.

The F77 credits Apple, Adobe, AT&T-IS, Motorola, SUN, CSRG, and Unisoft.

In the A/UX 0.7 build there is a 'greenhills' marker in crt0.o and libc so it looks like they used greenhills before switching (self hosting?) to pcc?