[PATCH] faster strlen implementation

[Dmitrieva Liubov]

> How important today is handle SLOW_BSF? Google search showed
> that K8 had slow bsf.

At the moment all versions marked as "for SLOW_BSF" are using just for
atom. But I care about Atom performance and if you don't have atom
machine I can help with regression testing.

> I also added sligthly faster version that uses sse4.1 instruction ptest instead pmovmskb
> testl pair.

Ptest has almost the same performance as pmovmskb + test
(Core i7:
ptest - latency 2, throughput 1
pmovmskb - latency 1, throughput 1
test -  latency 1, throughput 0.33)

It seems to me better to remove IFUNC wrapper at all and use just sse2
version if there are no regressions for atom without SLOW_BSF version
because we can eliminate IFUNC wrapper overhead.

--
Liubov Dmitrieva
Intel Corporation