libslc: Factor platform dependent signal handling
libslc: Use _signal_table in signal function (WIP)
libslc: Add _signal_action private function to register signals (WIP)
libslc: Rename _sigaction.c to _signal_action.c
libslc: Add _signal_action.c to Makefile
libslc: Fix _close system call + Add syscall_cp macros.