Revert "lib/string: use glibc version"

This reverts commit f57b68c217d1fb87bb78b7904ff71326612c9eae.
Revert "lib/rwsem.c: fix compatibility issues with Linux 3.0.y"

This reverts commit 62f0aa05bbc0507b4e014c6cc5a46cf98a02ee29.