cksum: use pclmul hardware instruction for CRC32 calculation

Use cpuid to detect CPU support for hardware instruction.
Fall back to slice by 8 algorithm if not supported.
A 500MiB file improves from 1.40s to 0.67s on an i3-2310M

* configure.ac [USE_PCLMUL_CRC32]: A new conditional,
set when __get_cpuid() and clmul compiler intrinsics are supported.
* src/cksum.c (pclmul_supported): A new function using __get_cpuid()
to determine if pclmul instructions are supported.
(cksum): A new function refactored from cksum_slice8(),
which calls pclmul_supported() and then cksum_slice8()
or cksum_pclmul() as appropriate.
* src/cksum.h: Export the crctab array for use in the new module.
* src/cksum_pclmul.c: A new module to implement using pclmul intrinsics.
* po/POTFILES.in: Reference the new cksum_pclmul module.
* src/local.mk: Likewise.  Note we build it as a separate library
so that it can be portably built with separate -mavx etc. flags.
* tests/misc/cksum.sh: Add new test modes for pertinent buffer sizes.

po/POTFILES.in

@@ -44,6 +44,7 @@ src/chown-core.c
 src/chown.c
 src/chroot.c
 src/cksum.c
+src/cksum_pclmul.c
 src/comm.c
 src/copy.c
 src/coreutils.c