[vectorapi]: Initial aarch64 backend implementation for FP16 operations

Add initial FP16 aarch64 backend implementation for both Neon and SVE for the FP16 operations whose c2 support has been added in commit - https://github.com/openjdk/panama-vector/commit/7460d9327aac7d1d2ba6aed4e7137a417dbf6a97