5291 x86 {high,low}bit rely on undefined behavior

*** 46,78 ****
  #endif
  
  extern __GNU_INLINE int
  highbit(ulong_t i)
  {
!         long value = -1l;
  
          __asm__(
!             "bsr" __SUF " %1,%0"
!             : "+r" (value)
!             : "r" (i)
              : "cc");
  
!         return ((int)(value + 1));
  }
  
  extern __GNU_INLINE int
  lowbit(ulong_t i)
  {
!         long value = -1l;
  
          __asm__(
!             "bsf" __SUF " %1,%0"
!             : "+r" (value)
!             : "r" (i)
              : "cc");
  
!         return ((int)(value + 1));
  }
  
  extern __GNU_INLINE uint_t
  atomic_btr32(uint32_t *memory, uint_t bitnum)
  {
--- 46,82 ----
  #endif
  
  extern __GNU_INLINE int
  highbit(ulong_t i)
  {
!         long value;
!         uint8_t zf;
  
          __asm__(
!             "bsr" __SUF " %2,%0;"
!             "setz %1"
!             : "=r" (value), "=q" (zf)
!             : "mr" (i)
              : "cc");
  
!         return (zf ? 0 : (value + 1));
  }
  
  extern __GNU_INLINE int
  lowbit(ulong_t i)
  {
!         long value;
!         uint8_t zf;
  
          __asm__(
!             "bsf" __SUF " %2,%0;"
!             "setz %1"
!             : "=r" (value), "=q" (zf)
!             : "mr" (i)
              : "cc");
  
!         return (zf ? 0 : (value + 1));
  }
  
  extern __GNU_INLINE uint_t
  atomic_btr32(uint32_t *memory, uint_t bitnum)
  {