cmake/tools: Fix panic_immediate_abort deprecation in Rust build

[no1wudi]

Summary

Fix the panic_immediate_abort compile error in Rust build system by replacing the deprecated -Zbuild-std-features=panic_immediate_abort flag with the new -Cpanic=immediate-abort compiler flag.

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

error: panic_immediate_abort is now a real panic strategy!
Enable it with `panic = "immediate-abort"` in Cargo.toml,
or with the compiler flags `-Zunstable-options -Cpanic=immediate-abort`.

This change updates both CMake and Makefile build systems to use the new approach via RUSTFLAGS environment variable.

Impact

• Impact on user: NO - No user-facing changes, only fixes build with newer Rust toolchains
• Impact on build: YES - Fixes Rust build with recent nightly compilers
• Impact on hardware: NO
• Impact on documentation: NO
• Impact on security: NO
• Impact on compatibility: YES - Maintains compatibility with both old and new Rust versions by using the officially supported flag

Testing

Build Host: Ubuntu 22.04, x86_64, Rust nightly 2025-03-01
Target: ARM stm32f746g-disco:nsh

Verified the fix by:

1. Enabling Rust hello example with cargo (CONFIG_EXAMPLES_HELLO_RUST_CARGO=y)
2. Enabling release mode (CONFIG_DEBUG_FULLOPT=y)
3. Enabling TLS support (CONFIG_TLS_NELEM=4) required for Rust std library
4. Building successfully without the panic_immediate_abort error

Build output:

[1123/1127] Building Rust crate hello
   Compiling hello v0.1.0
    Finished `release` profile [optimized] target(s) in 11.34s
[1124/1127] Linking CXX executable nuttx
Memory region         Used Size  Region Size  %age Used
           flash:      126020 B         1 MB     12.02%
           sram1:        5896 B       240 KB      2.40%
[1127/1127] Generating System.map

The firmware builds successfully and links the Rust static library without errors.

[cederom]

Thank you @no1wudi :-)