Require binutils 2.20 or later to build glibc

[Roland McGrath]

In its own right, I think 2.20 seems like a reasonable baseline.
But it seems suspect that we're requiring a binutils years newer
than the GCC we require.  Perhaps it would be OK to require 4.4.3
by now, which meshes better with 2.20.  But I'm not at all sure.

I vaguely recall people posting before that requiring too new
a binutils was burdensome for some platform or other, but I don't
recall the details.  I think we should at least get each arch maintainer 
to weigh in explicitly before deciding.

Independently, I think it's a bit dubious that we're requiring IFUNC
support unconditionally.  (I hadn't noticed we used it outside of
multiarch.)  Perhaps my tendencies move too slowly, but it still feels a
might newfangled.


Thanks,
Roland