additional fixes for linux kernel apis with old syscalls removed

src/process/vfork.c

 #define _GNU_SOURCE
 #include <unistd.h>
+#include <signal.h>
 #include "syscall.h"
 #include "libc.h"
 
 pid_t __vfork(void)
 {
 	/* vfork syscall cannot be made from C code */
+#ifdef SYS_fork
 	return syscall(SYS_fork);
+#else
+	return syscall(SYS_clone, SIGCHLD, 0);
+#endif
 }
 
 weak_alias(__vfork, vfork);

src/stat/fstat.c

@@ -14,7 +14,11 @@ int fstat(int fd, struct stat *st)
 
 	char buf[15+3*sizeof(int)];
 	__procfdname(buf, fd);
+#ifdef SYS_stat
 	return syscall(SYS_stat, buf, st);
+#else
+	return syscall(SYS_fstatat, AT_FDCWD, buf, st);
+#endif
 }
 
 LFS64(fstat);