[RFC] fixing extract_struct_value_address

[Michael Snyder <msnyder@redhat.com>]

Problem: Find a function's return value when it is a struct 
returned by reference (thru a pointer).

Solution level one: Take the value of the register that was
used by the caller to pass the struct return address.

Shortcoming: that register isn't preserved, so may be clobbered.

Solution level two: Save the struct_return address when it
is passed to store_struct_return (or push_arguments), and
recover it when it is needed by extract_struct_value_address.

Shortcoming: Not reentrant.  Nested function calls will clobber it.

Proposed solution: create a stack structure, and "push" the
struct_return address in store_struct_return, popping it in
extract_return_address.  If you can't find it on the stack, 
then use the value of the appropriate arg0 register.

I think this should work for most targets, so the code for
managing the stack can be shared.

What do you think?