Prevent compiler warnings in GC_FindTopOfStack and GC_ports (Darwin)

Prevent compiler warnings in GC_FindTopOfStack and GC_ports (Darwin)

* darwin_stop_world.c (GC_FindTopOfStack): Initialize "frame" local
variable from "stack_start" unless done via PPC lwz/ld instruction (to
prevent "uninitialized variable use" compiler warning;
add assertion for "stack_start" value; add 'U' suffix to int constant
to prevent comparison of signed and unsigned value.
* os_dep.c (GC_ports): Explicitly initialize "reply" field to zero if
THREADS (to suppress compiler warning on Darwin).