🔧 👷 Refactor LLVM toolchain for native builds and improved CI

[kammce]

Enable the LLVM toolchain package to build applications for native
platforms (Linux, macOS, Windows) in addition to cross-compilation
targets. Previously, the toolchain only configured itself when
settings_target was present (cross-compilation). Now it properly
detects and configures for native builds as well.

Key changes:

• Add comprehensive build environment variables (CC, CXX, AR, LD, NM,
  OBJCOPY, OBJDUMP, RANLIB, STRIP, ADDR2LINE, etc.) for better
  toolchain integration
• Add CMAKE_SYSTEM_NAME configuration for each platform to ensure
  CMake correctly identifies the target OS
• Extend CMake extra_variables with compiler and utility tool paths
• Refactor llvm-20 profile to use Jinja2 templating with host/build
  context separation and auto-detection of OS/arch
• Remove redundant platform-specific profiles (linux_arm, linux_x86_64,
  macos_arm, macos_x86_64, windows_arm, windows_x86_64)

CI workflow improvements:

• Add display_name parameter for clearer job naming in GitHub Actions
• Rename demo_profile to host_profile for semantic clarity
• Add workflow concurrency control to cancel in-progress runs
• Simplify verify workflow with separate native/cross build steps
• Add ninja build verification step using LLVM toolchain

Demo and test updates:

• Rename test_package to test_package_skip to prevent auto-execution
• Update demo conanfiles to explicitly require ninja and cmake
• Add CMakeToolchain generator configuration in demos
• Enable CMAKE_CXX_SCAN_FOR_MODULES and CMAKE_COLOR_DIAGNOSTICS

Documentation:

• Update README with the changes