rust-target-config: fix feature for vfpv4f16

Based on [1] which fixed the target for vfpv3d16. Adding the support for
vfpv4d16 with the same set of changes.

Tested with running tokio which hit coredump before the change. It
worked fine after this change.

[1] https://lists.openembedded.org/g/openembedded-core/message/185702



Signed-off-by: Willy Tu <wltu@google.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>