LLVM intrinsic fallback

[BradyPlanden]

Fall back to external functions when LLVM intrinsics are unavailable. I'm not sure what the best way to test this within the crate is. Any suggestions would be appreciated!

[martinjrobins]

Thanks @BradyPlanden, for a test I would just add a test for the tan function somewhere here:

diffsl/diffsl/src/execution/compiler.rs

Line 1446 in 4ffa31f

exp_function: "r { exp(2) }" expect "r" vec![f64::exp(2.0)],

.

This should fail for llvm < 19 before your fix, and pass afterwards.

[BradyPlanden]

Thanks @martinjrobins, I've tested on main locally and can confirm the additional tan test fails with cargo test --features llvm18-1