find /usr/bsd/var/tmp/pkgtools/pkg_install/work/pkg_install-20080627 -name audit.o ./admin/audit.oHm. That doesn't explain what you are seeing. It certainly works on NetBSD and I am somewhat puzzled that the same code doesn't compile here.
I seem to be getting a pkg_admin binary with the correct "--help" output, but which won't accept the "fetch-pkg-vulnerabilities" option. In what cases can this code-path be possible? I think you mentioned some reduced-functionality bootstrap mode where this would be left out?
I'm thinking that there must be some relatively simple explanation for this behaviour - so if we can find out what's going wrong (perhaps a hard-coded path or a test which unexpectedly fails on IRIX?) then it shouldn't be too much hassle to fix...
(Or even, could this be due to the fact that the IRIX linker doesn't re-order objects passed on the command-line, so if audit.o is passed too early then it could (possibly?) be omitted entirely?)
Cheers, Stuart