Remove legacy crosstool fields

For this to happen we need to:

1. Create a CcToolchainProvider Skylark API to replace existing ctx.fragments.cpp.compiler_options() and ctx.fragments.cpp.linker_options() etc.
   Done in Implement Skylark API for C++ toolchain/feature configuration #4571
2. Prepare migration tooling (elaborated [here]).(https://docs.google.com/document/d/1uv4c1zag6KvdI31qdx8C6jiTognXPQrxgsUpVefm9fM/edit#).
3. Add incompatible flags to help Bazel users migrate.
4. Migrate all crosstools to features.
5. Remove legacy fields from crosstools:
   • all supports_* fields
   • static_runtimes_filegroup
   • dynamic_runtimes_filegroup
   • compiler_flag
   • optional_compiler_flag
   • cxx_flag
   • optional_cxx_flag
   • unfiltered_cxx_flag
   • optional_unfiltered_cxx_flag
   • linker_flag
   • optional_linker_flag
   • dynamic_library_linker_flag
   • optional_dynamic_library_linker_flag
   • test_only_linker_flag
   • objcopy_embed_flag
   • ld_embed_flag
   • ar_flag
   • ar_thin_archives_flag
   • gcc_plugin_compiler_flag
   • compilation_mode_flags
   • lipo_mode_flags
   • linking_mode_flags
   • gcc_plugin_header_directory
   • mao_plugin_header_directory
   • tool_path