[libc] Implement setjmp/longjmp

Add a very simple setjmp/longjmp implementation. No destructors or other
cleanup is handled.

src/libraries/libc/arch/amd64/crt/crt0.s

@@ -1,3 +1,10 @@
+; This file is part of the C standard library for the jsix operating
+; system.
+;
+; This Source Code Form is subject to the terms of the Mozilla Public
+; License, v. 2.0. If a copy of the MPL was not distributed with this
+; file, You can obtain one at https://mozilla.org/MPL/2.0/.
+
 extern _GLOBAL_OFFSET_TABLE_
 
 extern main