cksum,wc: support disabling hardware acceleration at runtime

cksum,wc: support disabling hardware acceleration at runtime

This is useful to give better test coverage at least,
and may be useful for users to tune their environment.

* bootstrap.conf: Reference the cpu-supports gnulib module.
* src/cksum.c: Use cpu_supports() rather than __builtin_cpu_supports().
* src/wc.c: Likewise.
* tests/cksum/cksum.sh: Adjust to testing all implementations.
* tests/wc/wc-cpu.sh: A new test to do likewise.
* tests/local.mk: Reference the new wc test.