RFC & patch: Rework MIPS command-line handling

[Richard Sandiford]

cgd@broadcom.com writes:
> say mips-elf builds multilibs for -mips1/-mips3...
> 
> you want -march=r4000 if specified on the command line to get -mips3.
> actually, you also want -march=r5000 to get -mips3, if -mips1 and
> -mips3 are your only choices...

And if you use -march=r5000 -mgp32, you want the -mips1 multilibs...

Not doing implicit ABI changes would make the multilibs easier in this
case.  Say we have -mabi=32/-mabi=o64.  If there was no implicit ABI
selection, you'd have to use -mabi=o64 on the command line to get o64
code, which would also mean you'd link against the o64 multlib.

That'd also be an argument in favour of -mabi=eabi64 and the like.

If there's only one way of setting the ABI (barring synonyms like
-32, which are easy to handle), then you're more likely to get
the right multilib.

Richard