Disable legacy C++ toolchain API

[hlopko]

Incompatible change.

We have deprecated the cc_toolchain Starlark API returning legacy CROSSTOOL fields. These methods are removed:

• compiler_options
• c_options
• cxx_options
• unfiltered_compiler_options
• link_options
• dynamic_link_options
• fully_static_link_options
• mostly_static_link_options

Use the new API from cc_common instead.
See full migration docs for instructions.

[hlopko]

Flag is flipped, therefore closing the issue.