sleep: tiny code shrink

sleep: tiny code shrink

...or rather, it WILL BE code shrink when gcc become clever enough
to not emit a second, useless XORing of ebx:

31 db                  xor    %ebx,%ebx
85 c0                  test   %eax,%eax
74 11                  je     73 <__GI_sleep+0x73>
31 db                  xor    %ebx,%ebx    <=== ?!

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>