Add support for runtime fallback to C

[hanno-becker]

Issue: At the moment, backends are statically enabled/disabled at compile-time. This is not for consumers requiring generic builds that are required to work on a diverse set of target CPUs with varying feature sets. AWS-LC is one such consumer. Specifically, single x86_64 / AArch64 builds are needed which work regardless of whether the host CPU supports, e.g., AVX2/AVX-512/AArch64+Crypto/SHA3 or other extensions.

Feature request: Allow consuming libraries to specify a runtime check guarding a backend or individual backend functions, and fall back to the default C implementation if the check fails. This check would typically be implemented as a CPU feature check.

[hanno-becker]

Done via #1148