lib: add weak clz/ctz functions ...

This is a copy of the Linux kernel implementation. This adds weak
functions for the clz and ctz gcc builtins. Normally GCC will map
those builtins to CPU instructions directly, but for CPUs where those
are unavailable, we need to provide a fallback implementation.

Fixes build of the zlib decompressor on ARMv4 CPUs.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>