[Bug target/119626] On aarch64, use the bfcvt instruction to cast to __bf16 when target supports it

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119626

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Note there is no such thing as arm64, it is aarch64 which is part of
armv8/9-a/r.