LibC: Implement setjmp() and longjmp().

I think this is correct. The following registers are saved and restored:
EBX, ESI, EDI, EBP, ESP and EIP.

LibC/setjmp.cpp

@@ -2,13 +2,32 @@
 #include <assert.h>
 #include <Kernel/Syscall.h>
 
-int setjmp(jmp_buf)
-{
-    //assert(false);
-    return 0;
-}
+asm(
+"setjmp:\n\
+    movl %ebx, 0(%eax)\n\
+    movl %esi, 4(%eax)\n\
+    movl %edi, 8(%eax)\n\
+    movl %ebp, 12(%eax)\n\
+    movl %esp, 16(%eax)\n\
+    movl (%esp), %ecx\n\
+    movl %ecx, 20(%eax)\n\
+    xorl %eax, %eax\n\
+    ret\n\
+");
 
-void longjmp(jmp_buf, int)
-{
-    assert(false);
-}
+asm(
+"longjmp:\n\
+    xchgl %edx, %eax\n\
+    test %eax, %eax\n\
+    jnz 1f\n\
+    incl %eax\n\
+1:\n\
+    mov 0(%edx), %ebx\n\
+    mov 4(%edx), %esi\n\
+    mov 8(%edx), %edi\n\
+    mov 12(%edx), %ebp\n\
+    mov 16(%edx), %ecx\n\
+    mov %ecx, %esp\n\
+    mov 20(%edx), %ecx\n\
+    jmp *%ecx\n\
+");