rust/hello: Optimize the build flags

Summary:
- Added `codegen-units = 1` and `opt-level = 'z'` to the release profile in `Cargo.toml`
- These changes optimize the build for minimal binary size

Impact:
- Reduces the final binary size by ~7% (244316 -> 228380 bytes)
- Improves resource utilization for embedded systems
- No functional changes to the application behavior

Signed-off-by: Huang Qi <huangqi3@xiaomi.com>

examples/rust/hello/Cargo.toml

@@ -9,11 +9,11 @@ crate-type = ["staticlib"]
 [profile.dev]
 panic = "abort"
 
-# Special hanlding for the panic! macro, can be removed once
-# the libstd port for NuttX is complete.
 [profile.release]
 panic = "abort"
 lto = true
+codegen-units = 1
+opt-level = 'z'
 
 [dependencies]
 serde = { version = "1.0", features = ["derive"] }