Add LLVM/Clang toolchain support with auto-detection

[G36maid]

Add automatic detection and configuration for LLVM/Clang toolchain alongside existing GCC support. The build system now detects which compiler is available and configures appropriate flags and tools.

• Auto-detect LLVM/Clang via CC_IS_CLANG variable
• Add Clang-specific flags (--target, warning suppressions)
• Use LLVM binutils (ld.lld, llvm-objdump, etc.) when Clang detected
• Refactor to use common ARCH_FLAGS for consistency
• Maintain backward compatibility with GCC toolchain

Part of #12

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Summary by cubic

Adds LLVM/Clang toolchain support with auto-detection for the RISC-V build. Automatically selects Clang or GCC and configures the correct flags and binutils, progressing Linear #12 (cross-toolchain builds).

• New Features

  • Auto-detect LLVM/Clang via CC_IS_CLANG (user can override).
  • Switch to LLVM tools when Clang is used (clang, ld.lld, llvm-objdump/readelf/objcopy/size).
  • Add Clang flags: --target=riscv32-unknown-elf and -Wno-unused-command-line-argument.

• Refactors

  • Introduce ARCH_FLAGS and apply to C/AS; unify LDFLAGS; keep GCC path intact.
  • Keep CROSS_COMPILE overridable (default riscv-none-elf-).

[cubic-dev-ai]

No issues found across 1 file

[jserv]

Thank @G36maid for contributing!