Support bits/syscall.h build for triarch systems

[Roland McGrath]

> Rather than restricting it to predefines needing -U I'd rather it lists 
> all the compiler options needed to select the relevant ABI (which might be 
> some -D and -U options, or might be a compiler option to select the ABI 
> directly).  

Sure.  What I described was just meant as an example of how we might have a
common rule with appropriate configuration via variables.


Thanks,
Roland