bsd.compiler.mk: detect Apple Clang for cross-builds

Apple clang uses a different versioning scheme, so if we enable or
disable certain warnings for Clang 11+, those might not be supported
in Apple Clang 11+. This adds 'apple-clang' to COMPILER_FEATURES, so that
bootstrap tools Makefiles can avoid warnings on macOS.

Reviewed By: imp
Differential Revision: https://reviews.freebsd.org/D29680