In our unit test, let us rely on our own reference
implementation using the reflected
CRC-32 ISO 3309 and CRC-32C polynomials. Let us also
test with various lengths.

Let us refactor the CRC-32 and CRC-32C implementations
so that no special compilation flags will be needed and
that some function call indirection will be avoided.

pmull_supported: Remove. We will have pointers to two separate
functions crc32c_aarch64_pmull() and crc32c_aarch64().