cmake/tools: Fix panic_immediate_abort deprecation in Rust build

Replace deprecated -Zbuild-std-features=panic_immediate_abort with
-Cpanic=immediate-abort compiler flag via RUSTFLAGS.

The panic_immediate_abort feature has been stabilized as a real panic
strategy in recent Rust nightly versions. The old method of enabling
it via -Zbuild-std-features is no longer supported and triggers a
compile error in core/src/panicking.rs.

Changes:
* cmake/nuttx_add_rust.cmake: Use RUSTFLAGS with -Cpanic=immediate-abort
* tools/Rust.mk: Use RUSTFLAGS with -Cpanic=immediate-abort

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

tools/Rust.mk

@@ -90,8 +90,8 @@ endef
 ifeq ($(CONFIG_DEBUG_FULLOPT),y)
 define RUST_CARGO_BUILD
 	NUTTX_INCLUDE_DIR=$(TOPDIR)/include:$(TOPDIR)/include/arch \
+    RUSTFLAGS="-Zunstable-options -Cpanic=immediate-abort" \
     cargo build --release -Zbuild-std=std,panic_abort \
-    -Zbuild-std-features=panic_immediate_abort \
 		--manifest-path $(2)/$(1)/Cargo.toml \
 		--target $(call RUST_TARGET_TRIPLE)
 endef