guard rtc syscall
native fix cpu_switch_context_exit
fix native ctxt switches

fix disableIRQ (remove context switch)
fix enableIRQ (make context switch delay-safe)
change interrupts-off signal-catch behaviour from error to warning
add context switch memory location warning
Merge pull request #229 from LudwigOrtmann/native_fix_syscall_segfault

fix some segfault sources in native