x86_64: ensure that dtor pointer passed to app is 0 so it won't be used

leaving it uninitialized caused unpredictable crashes or worse due to
calling an indeterminate function pointer.

src/ldso/x86_64/start.s

@@ -5,4 +5,5 @@ _start:
 	lea 8(%rsp),%rsi
 	lea _GLOBAL_OFFSET_TABLE_(%rip),%rdx
 	call __dynlink
+	xor %edx,%edx
 	jmp *%rax