feat(build): preventing interference from OUTLINED_FUNCTION_* symbols

[zouguangxian]

This pull request introduces improvements to the build process for the syscall-cycles example and enhances how Rust compiler flags (RUSTFLAGS) are handled in the build system. The main focus is to ensure more accurate cycle counts during benchmarking and to allow for better customization of build flags.

Build process improvements:

• Disabled the LLVM machine outliner in release builds of the syscall-cycles example by adding -Cllvm-args=-enable-machine-outliner=never to RUSTFLAGS, preventing interference from OUTLINED_FUNCTION_* symbols in cycle counts.

Build system enhancements:

• Updated the build_binary_with_rustflags function to append any flags set in the RUSTFLAGS environment variable to the list of compiler flags, allowing for more flexible and user-configurable builds.