count-leading-zeros: new module

count-leading-zeros: new module

I needed gcc's clz to determine the most significant bit of a
number (useful for things like truncating to a power of 2),
and was surprised it is not a standardized function (the
opposite direction of finding the least significant bit is
given by ffs).  This borrows heavily from the design of the
count-one-bits module.

* modules/count-leading-zeros: New module.
* m4/count-leading-zeros.m4: New file.
* lib/count-leading-zeros.h: Likewise.
* modules/count-leading-zeros-tests: New test.
* tests/test-count-leading-zeros.c: New file.
* MODULES.html.sh (Integer arithmetic functions): Document it.

Signed-off-by: Eric Blake <eblake@redhat.com>