ir_aarch64.c contains tests that require extensions such as FP16

The tests in `ir_aarch64.c` that require extensions such as FP16 should presumably be moved to other files such as `ir_aarch64_v82.c`.

This isn't just for tidyness: `code_api|api.ir` currently fails on hardware without FP16. I thought it would be possible to make it pass by inserting a call to `enable_all_test_cpu_features()` but that turned out to be more difficult than I expected: I think the test is not linked with the right library for that.

I have a PR (#6433) that move the LDAPR tests out of `ir_aarch64.c`. After that I think the only feature required by that file is FP16. I think there is also a false dependency on FP16 due to #6432.