Fix PR gas/827

[Ian Lance Taylor]

Alan Modra <amodra@bigpond.net.au> writes:

> While poking at pseudo_set, I noticed that assignment to section symbols
> is only prevented when assigning some other symbol to the section
> symbol.  This seemed odd to me, so I modified the code to give an error
> on any assignment to section symbols.  Another oddity is special casing
> sym1 = sym2 + const for const == 0.  When const != 0, the old code
> created an expression symbol, and I can't see the reason for that.  Ian,
> do you know why this was done?

No, I don't.  It's particularly odd since the subsequent code is
careful to add in exp.X_add_number.

I've often wished that I had taken a copy of the Cygnus CVS repository
when I left.  I had access for a long time after I left, then I
suddenly lost it.

Ian