fix: suppress warnings about unused llvm flags

[rioam2]

This pull request updates the initialize_wasi_toolchain function in wasi-sdk.toolchain.cmake to improve compatibility with older versions of clang and clarify linker flag usage. Ensures that certain -mllvm flags do not trigger unused argument warnings, and library linking for exception and setjmp/longjmp support is made more explicit.

• Wrapped -mllvm flags with --start-no-unused-arguments and --end-no-unused-arguments in both CMAKE_C_FLAGS and CMAKE_CXX_FLAGS to avoid unused argument warnings in older clang versions. [1] [2]
• Moved -fwasm-exceptions to only be appended to CMAKE_CXX_FLAGS and clarified linking against libunwind for exception handling by using -lunwind in CMAKE_EXE_LINKER_FLAGS.
• Clarified linking against the setjmp library for setjmp/longjmp support by using -lsetjmp in CMAKE_EXE_LINKER_FLAGS.

[Copilot]

Pull request overview

This PR adjusts initialize_wasi_toolchain() to reduce “unused argument” warnings from clang when passing WebAssembly-related -mllvm flags, and to make exception / setjmp linkage more explicit via linker flags.

Changes:

• Wraps -mllvm WebAssembly backend flags with --start-no-unused-arguments / --end-no-unused-arguments for both C and C++ flags.
• Moves exception enablement to CMAKE_CXX_FLAGS and moves libunwind linkage to CMAKE_EXE_LINKER_FLAGS.
• Moves setjmp linkage to CMAKE_EXE_LINKER_FLAGS and applies the same unused-argument suppression wrapping for SJLJ-related -mllvm flags.

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