Use __builtin_clz() supported by GCC and Clang to figure out
how many bits we should shift instead of shifting by a bit
in a loop until the value gets normalized. Improves performance
of this function by up to 3x in worst-case scenario and overall
straw2 performance by ~10%.

Reflects ceph.git commit 110de33ca497d94fc4737e5154d3fe781fa84a0a.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>