Add support for runtime builtins

[Fernthedev]

Referencing the original issue here, rust-lang/compiler-builtins#468, it seems some builtins for Android aren't included properly. This adds flags that links the proper builtins statically.

The default clang version constant is undesirable but I'm not sure how else to do it.

[Fernthedev]

As the MacOS M1 CI failed (and likely the Linux one will too) I'll fix it once I test this on the Mac. I suspect it's due to the linker include path being different than what I originally anticipated.

Tested on Windows originally.

[bbqsrc]

The version of clang in NDK 26 is 17. You'll need to use a function to find the highest version available of the clang libraries if we want this to be maintainable in the long run.

[bbqsrc]

@Fernthedev Merged it because it works, but we avoid using RUSTFLAGS as much as possible because it doesn't lead to good things overall. I would like that the CLI has a flag like --link-builtins that optionally enables this functionality, before I put it into a release. What do you think?

[Fernthedev]

I see. Would it be preferable if this was under a flag similar to bindgens? --builtins perhaps?

[bbqsrc]

--link-builtins.