patch by njk, simplifies thread register initialization for x86_64

It's not necessary to save any registers on the stack across syscall in
x86_64 __set_thread_area.  Don't waste cycles or bytes on it.

src/thread/x86_64/__set_thread_area.s

@@ -3,13 +3,9 @@
 .global __set_thread_area
 .type   __set_thread_area,%function
 __set_thread_area:
-        push %rbx               /* save x86_64 abi clobbered registers */
-        push %r11
         mov %rdi,%rsi           /* shift for syscall */
         movl $0x1002,%edi       /* SET_FS register */
         movl $158,%eax          /* set fs segment to */
         syscall                 /* arch_prctl(SET_FS, arg)*/
-        pop %r11                /* restore clobbered registers */
-        pop %rbx
 	ret
 .size __set_thread_area,.-__set_thread_area