darwin --m=32: fix Parrot_sysmem_amount GH #1113

Need to use HW_PHYSMEM there. No chance to get >2GB RAM in i386.
This syscall worked on darwin/ppc32 also before.
Avoid backtrace within an sysmem exception GH #1113

Protect too early exception call via Parrot_default_encoding_ptr check.
This should be calling PANIC, but we do not want to dump core here.
sysctl err msg and location should be enough.