Tighten up GDB's support for returning structs by value.

Tighten up GDB's support for returning structs by value.
* s390-tdep.c (s390_use_struct_convention): New function.
(s390_gdbarch_init): Register it as the S/390's
USE_STRUCT_CONVENTION method.  Register
generic_cannot_extract_struct_value_address as our
EXTRACT_STRUCT_VALUE_ADDRESS method.
* arch-utils.c (generic_cannot_extract_struct_value_address): New
function.
* arch-utils.h: Add corresponding prototype.