This implementes another change in ELFv2: the stack frame no longer
contains the reserved double words for linker and compiler use
(which weren't really used for much of anything anyway). This
affects placement of on-stack parameters in inferior calls.
gdb/ChangeLog:
* ppc-sysv-tdep.c (ppc64_sysv_abi_push_dummy_call): Use correct
offset to the stack parameter list for the ELFv2 ABI.