[rfa:ppc64] Use target in convert_from_func_ptr_addr

[Andrew Cagney]

> I'm trying to understand where you're going with these changes.

See my most recent posts to gdb@, along with the archives.  There is a 
long standing and oft repeated goal of cleaning up the target stack.  In 
part making it more OO.

> Will a GDB user see enhanced functionality as a
> result?

I guess so.  At present FSF's GDB when debugging shared libraries on 
GNU/Linux does this:

(top-gdb) run
Starting program: 
/home/cagney/PENDING/YYYY-MM-DD-target-convert-func/64/gdb/gdb
Warning:
Cannot insert breakpoint -1.
Error accessing memory address 0x7fdffec35c: Input/output error.
(top-gdb)

Andrew