Remove Apple RNG fallbacks and simplify implementation

[BlackHoleFox]

Now that we have higher Apple platform requirements, the RNG code can be simplified a lot. Since getentropy still doesn't look to be usable outside macOS this implementation:

• Removes any macOS fallback paths and unconditionally links to getentropy
• Minimizes the implementation for everything else (iOS, watchOS, etc).

CCRandomGenerateBytes was added in iOS 8 which means that we can use it now. It and SecRandomCopyBytes have the exact same functionality, but the former has a simpler API and no longer requires libstd to link to Security.framework for one function. Its also available in all the other target's SDKs.

Why care about getentropy then though on macOS? Well, its still much more performant. Benchmarking shows it runs at ~2x the speed of CCRandomGenerateBytes, which makes sense since it directly pulls from the kernel vs going through its own generator etc.

Semi-related to a previous, but reverted, attempt at improving this logic in #101011