Dear Gale,
I think the gcc approved way of doing that is to is to use an external specs-file with the -specs=specs-file gcc/gpc command line parameter option to override the built-in assembler invoking driver spec.
My gpc is using this specs file by default:
/Developer/Pascal/gpc346u4/lib/gcc/i386-apple-darwin10/3.4.6/specs
First start off by seeing what the current built-in specs look like with gpc -dumpspecs
gpc -dumpspecs > specs
produces a local specs file identical to the default one, and containing exactly this line, even unto the space before "as".
*invoke_as: %{!S:-o %|.s | as %(asm_options) %m.s %A }
I can change the default specs file to point to my GNU assembler, or the new file, or a small file containing only the above, which is nice.
Copy that into a new file that you'll use for the -specs= parameter option.
That works. Thank you.
By the way: I tried to move the clang assembler and insert a symbolic link to the GNU assembler in its place. But OS 10.12.6 won't let me do anything to /usr/bin/as. Not even as root.
Best Wishes, Kevan