cmp: tune 'cmp a b' for GCC x86

Performance problem reported by David Balažic in:
http://lists.gnu.org/archive/html/bug-diffutils/2013-08/msg00013.html
* src/system.h (word): Make it size_t, not uintmax_t.
This sped up plain cmp 90% on my tests (GCC 4.8.1, x86).
cmp, diff, sdiff: tune by using rawmemchr

On my platform (AMD Phenom II X4 910e, Fedora 17 x86-64), this sped up
'cmp -n 8GiB /dev/full /dev/zero' by a factor of 3.8, and
'cmp -sn 8GiB /dev/full /dev/zero' by a factor of 1.8.
* bootstrap.conf (gnulib_modules): Add rawmemchr.
* src/cmp.c (cmp): Optimize the common case where buffers are the same,
by using count_newlines rather than block_compare_and_count.
(block_compare_and_count): Remove.
(count_newlines): New function.
* src/cmp.c (count_newlines):
* src/io.c (prepare_text):
* src/sdiff.c (lf_copy, lf_skip, lf_snarf):
Use rawmemchr instead of memchr, for speed.