spawn/Init: allow system call "sigaltstack" for libasan

src/spawn/Init.cxx

@@ -103,6 +103,9 @@ LimitSysCalls(FileDescriptor read_fd, pid_t kill_pid)
 	   ignores this (to prevent SIGKILL) */
 	sf.AddRule(SCMP_ACT_ERRNO(ENOMEM), SCMP_SYS(brk));
 
+	/* needed by libasan (if AddressSanitizer is enabled) */
+	sf.AddRule(SCMP_ACT_ALLOW, SCMP_SYS(sigaltstack));
+
 	sf.Load();
 }