[AArch64][ARM] Make neon fp16 generic intrinsics always available. (#87467)

By generic intrinsics this mean things like dup, ext, zip and bsl that
can always be executed with integer s16 operations and do not require
fullfp16. This makes them always available, and brings them inline with
GCC.
https://godbolt.org/z/azs8eMv54

The relevant test cases have been moved into their own files, to allow
them to be tested with armv8-a and armv8.2-a+fp16.