Fix configure logic for disabling multi-arch

[Roland McGrath]

I think that's close enough to obvious.

Incidentally, I think that test should try % before @.
AFAIK % is the one choice that works on all the machines.

We have some cases like x86_64/{time,gettimeofday}.c that use %
unconditionally.  

It might be reasonable to drop ASM_TYPE_DIRECTIVE_PREFIX now and instead
just have a sanity check that % does indeed work.  But I don't really know
the historical or all-machines state of binutils for that.


Thanks,
Roland