* i386-tdep.c (i386_extract_return_value,

* i386-tdep.c (i386_extract_return_value,
i386_store_return_value): Add gdbarch argument.  Use it instead of
determining it from REGCACHE.  Remove recursive call for
structures with a single member.
(i386_use_struct_convention): Remove.
(i386_reg_struct_return_p): New function.
(i386_return_value): New function.
(i386_gdbarch_init): Set return_value, don't set
extract_return_value, store_return_value and
use_struct_convention.
* x86-64-tdep.c (x86_64_init_abi): Set return_value to NULL.