lib: add weak clz/ctz functions

Some architectures need __c[lt]z[sd]i2() for __builtin_c[lt]z[ll] and It
causes build failure. They can be implemented using the fls()/__ffs() and
overridden by linking arch-specific versions may not be implemented yet.
This is required by "lib: add lz4 compressor module".

Reference: https://lkml.org/lkml/2013/4/18/603
Signed-off-by: Chanho Min <chanho.min@lge.com>
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: "Darrick J. Wong" <djwong@us.ibm.com>
Cc: Bob Pearson <rpearson@systemfabricworks.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Herbert Xu <herbert@gondor.hengli.com.au>
Cc: Yann Collet <yann.collet.73@gmail.com>
Cc: Kyungsik Lee <kyungsik.lee@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Modified by Locasto for 3.0.xx
Signed-off-by: Locasto <amperific@gmail.com>