ANDROID: rust: disable floating point target features

When switching to a target.json, I enabled all features that existed on
the Rust target by default. However, I did not notice that we explicitly
disable the neon feature. So, disable the neon feature.

Also disable fp-armv8. Note that in all cases other than target.json,
disabling neon implicitly disables fp-armv8.

Fixes: 3396c213 ("ANDROID: rust: use target.json for aarch64")
Change-Id: I7f9ad6044df34101e9dcdd6673968cc431628253
Signed-off-by: Alice Ryhl <aliceryhl@google.com>