coverage of the accelerated CRC implementation and some differences
bewteen the internal and external implementation.

This change adds CI coverage to the
linux_clang-latest_libstdcxx_bazel.sh script assuming this script
always runs on machines of at least the Intel Haswell generation.

Fixes include:
 * Remove the use of the deprecated xor operator on crc32c_t
 * Remove #pragma unroll_completely, which isn't known by GCC or Clang:
   https://godbolt.org/z/97j4vbacs
 * Fixes for -Wsign-compare, -Wsign-conversion and -Wshorten-64-to-32

PiperOrigin-RevId: 491965029
Change-Id: Ic5e1f3a20f69fcd35fe81ebef63443ad26bf7931