Handle function descriptors in call_site_target

Handle function descriptors in call_site_target

call_site_target::iterate_over_addresses may look up a minimal symbol.
On platforms like PPC64 that use function descriptors, this may find
an unexpected address.  The fix is to use gdbarch_convert_from_func_ptr_addr
to convert from a function descriptor to the address recorded at the
call site.

I've added a new test case that is based on the internal AdaCore test
that provoked this bug.  However, I'm unable to test it as-is on
PPC64.