Add missing register preserve in x86/x86_64 longjmp.

Change-Id: I3fc442f819229640e9c0ec4f9c865d9bf0e70881
diff --git a/libc/arch-x86_64/bionic/setjmp.S b/libc/arch-x86_64/bionic/setjmp.S
index 09d61f5..56ebb07 100644
--- a/libc/arch-x86_64/bionic/setjmp.S
+++ b/libc/arch-x86_64/bionic/setjmp.S
@@ -159,8 +159,10 @@
 
   // Check the cookie.
   pushq %rax
+  pushq %r11
   movq %rcx, %rdi
   call PIC_PLT(__bionic_setjmp_cookie_check)
+  popq %r11
   popq %rax
 
   // Return 1 if value is 0.