Fix signal related issues

Fix signal related issues

1. We enable _POSIX_REALTIME_SIGNALS preprocessing flag in order to use
siginfo_t definition.

2. We extend the siginfo_t definition with si_addr field by patching original
code. Further more, we use the same definition of struct sigaction as in the
case of RTEMS.

3. We have a circular inclusion cycle in the original code, where time.h
includes signal.h and viceversa. We fix this by using forward declaration in the
original code (again, by patching).

Signed-off-by: Costin Lupu <costin.lupu@cs.pub.ro>
Reviewed-by: Charalampos Mainas <charalampos.mainas@neclab.eu>