[patch/rfc] New (!) BFD target

[Andrew Cagney]

> Also, how does this interact with the "exec" target, whose
> functionality it seems to duplicate in some ways?  There should be an
> exec target down the stack somewhere, why aren't you using that?

bfd-target.h is "new" and "re-entrant".  "exec.c" is "old" and "very 
very non-re-entrant".  Recall that I wrote:

> What you're casually dismissing as trivial: "Not everywhere will need to be converted" and "Eventually, with low urgency, the non-ops should be moved out of it" are exactly the things I also need *now*.

The change I posted, for svr4-solib.o, requires two active bfd-backed 
targets (the executable and ld.so), and hence re-entrency[sp].

Follow on changes can see "corefile.c" and "exec.c" both overhauled to 
use this new bfd-target foundation/parent object.

Andrew