Make bitpos() handle arguments with many set bits, no GENERIC

[dougm]

Summary

When bitpos can't be implemented with an inline ffs* instruction,
change the binary search so that it does not depend on a single bit
only being set in the bitmask. Use bitpos more generally, and avoid
some clearing of bits to accommodate its current behavior.

[dougm]

Maybe this undo/redo was unnecessary, but when I checked in the quick fix, I got a *reject* email about the lack of a reviewer, so I assumed the checkin didn't happen. Maybe that's wrong.

[kib]

So _Generic did not worked because of gcc 4.2.1. It might require some time for 4.2.1 to be expunged on tier2.