build: add caching for involved configure checks

build: add caching for involved configure checks

* configure.ac: Wrap the following with AC_CACHE_VAL,
so that they can be cached / overridden.  We use
the "utils_cv_" prefix as they're coreutils specific overrides.
utils_cv_avx2_intrinsic_exists,
utils_cv_brain_16_bit_supported,
utils_cv_ieee_16_bit_supported,
utils_cv_pclmul_intrinsic_exists,
utils_cv_stdbuf_supported.