FYI: make current_objfile local to symbol readers

[Mark Kettenis]

> 
> I'm checking this in on the trunk.
> 
> The global 'current_objfile' is used by some symbol readers, but only in
> a local way.  This patch removes makes this global be local to the
> readers which use it.  I think this is clearer and also prevents new
> (bad) uses of this global in the future.
> 
> Built and regtested on the buildbot.

But isn't it a bad idea to have multiple static variables with the
same name?  It makes inspecting the variable with GDB a bit more
difficult...