ci: Fix an error in cargo check

This commit fixes the below error in CI's rule-checker workflow.

```
$ cargo check --workspace  --exclude islet_sdk --exclude rsi_el0 --exclude cose --exclude cca_token
...
error: register class `vreg` requires the `neon` target feature
   --> /home/changho/.cargo/registry/src/index.crates.io-6f17d22bba15001f/half-2.3.1/src/binary16/arch/aarch64.rs:171:9
    |
171 |         in(vreg) a,
    |         ^^^^^^^^^^

error: register class `vreg` requires the `neon` target feature
   --> /home/changho/.cargo/registry/src/index.crates.io-6f17d22bba15001f/half-2.3.1/src/binary16/arch/aarch64.rs:172:9
    |
172 |         in(vreg) b,
    |         ^^^^^^^^^^

error: could not compile `half` (lib) due to 32 previous errors
```

Signed-off-by: Changho Choi <[email protected]>

.cargo/config

@@ -1,3 +1,8 @@
 [build]
 target = "aarch64-unknown-none-softfloat"
 target-dir = "out"
+
+[target.aarch64-unknown-none-softfloat]
+rustflags = [
+  "-C", "target-feature=+neon",
+]