BUG/MEDIUM: tools: fix direction of my_ffsl()

BUG/MEDIUM: tools: fix direction of my_ffsl()

Commit 27346b01a ("OPTIM: tools: optimize my_ffsl() for x86_64") optimized
my_ffsl() for intensive use cases in the scheduler, but as half of the times
I got it wrong so it counted bits the reverse way. It doesn't matter for the
scheduler nor fd cache but it broke cpu-map with threads which heavily relies
on proper ordering.

We should probably consider dropping support for gcc < 3.4 and switching
to builtins for these ones, though often they are as ambiguous.

No backport is needed.