[ARM] Add missing target for Arm neon test case.

This is a follow-up from https://reviews.llvm.org/D61717. Where Richard
described the issue with compiling arm_neon.h under
-flax-vector-conversions=none. It looks like the example reproducer does
actually work but what was missing was a test entry for that target.

Differential Revision: https://reviews.llvm.org/D88546