Fix backwards compatible system property structure

The original structure included four reserved 32-bit values. This
change adds these back into the structure so that the
__system_property_find_compat function will (again) process the system
properties correctly.
Allocate additional space on stack for indirect syscall

The caller is only required to allocate 16 bytes on the
stack for a0-a3. syscall is handling up to 6 arguments so
additional space is needed on the stack to avoid corrupting the
callers frame.

Change-Id: I054b31696decc3e17d9c70af18cd278b852235d1