[SVE] Adding codegen tests for SVE (#14239)

* [SVE] Adding codegen tests for SVE

Now that ci_arm contains LLVM 15, add tests to check that code for SVE can be correctly generated.

Add tests to cover a selection of arithmetic ops and gather loads. The gather load is currently xfailed until work is completed to enable them through TVM and LLVM.

* Reworking so dtype is passed as a pytest param

Author: neildhickey