How does one compile 64-bit executables on HP-UX PA-RISC?

[Dr. David Kirkby]

I have an HP C3600 workstation

http://www.b2net.co.uk/hp/hp_visualize_c3600_workstation.htm

which is fitted with 552 MHz PA-8600 PA-RISC CPU (64-bit) and 8 GB RAM.

If I use gcc -m64, I get an error.

drkirkby@hpbox:~$ gcc -m64 test.c
cc1: error: unrecognized command line option "-m64"

Can anyone tell me how to build 64-bit binaries with gcc on this machine?

Dave