Revert "Reimplement strlen"

This reverts commit 710e45c4b8539d028877769f1a4ec088c48fb5f1.

It breaks for some corner cases on big endian ppc64.
Given the stage of the release process it is best to revert for now.

Reported by: jhibbits

(cherry picked from commit 33f0540b13d949c7cc226a79927ddc2062ff98bf)