[PATCH] improve equate handling

[Nick Clifton]

Hi Jan,

> This adjusts equate handling by
> - allowing true forward references (which will always assume the
> referenced
>   symbols have at the point of use) through the new .eqv pseudo-op and
> the
>   new == operator
> - disallowing changing .equiv-generated equates (so that the protection
> this
>   provides is both forward and backward)
> - snapshotting equates when their value gets changed so that previous
> uses
>   don't get affected by the new value.
> - allowing expressions in places where absolute expressions (or
> register
>   names) are needed which were not completely resolvable at the point
> of
>   their definition but which are fully resolvable at the point of use
> 
> In addition it fixes PR/288.

I have approved and applied your patch.

In testing it I found that you had missed one backend where the expr() 
function was being called (in arc_parse_cons_expression in tc-arc.c), so 
I fixed that, and I also found a few more targets which do not support 
the BFD_RELOC_8 and so fail some of the new tests.  I updated the xfail 
expression in all.exp to account for these.

Cheers
   Nick