LibC: add needed stubs to build executables with our compiler

libc/include/unistd.h

@@ -6,16 +6,17 @@
 #define STDOUT_FILENO 1
 #define STDERR_FILENO 2
 
-// fork(), execv(), execve(), execvp(), getpid()
-
 __BEGIN_DECLS
 
+[[noreturn]] void _exit(int);
+
 pid_t fork(void);
+pid_t getpid(void);
 
 int execv(const char*, char* const[]);
 int execve(const char*, char* const[], char* const[]);
 int execvp(const char*, char* const[]);
 
-pid_t getpid(void);
+long syscall(long, ...);
 
 __END_DECLS