This seems to be a chicken-and-egg problem, but there are solutions.
 
If you have access to a Sun computer you can use it as a cross-compiler
to compile vmlinux for your target pmac machine.  You can get a Sun cross
compiler from ftp://ftp.nmt.edu:/pub/people/cort/
 
Another option is to use a cross compiler that runs within MacMiNT which
is a unix-like environemnt that runs like an application on any PowerPC
MacOS machine.  A complete system that's ready to compile linux-pmac with
little more than a "make vmlinux" command is now available from:
ftp://ftp.linuxppc.org/users/harry It is in Macbinary II format, so using
Fetch is a good way to retrieve it.
 
harry@linuxppc.org