Title: bazel test should exclude all blacklisted extensions/dependencies

Description:
Currently we are excluding blacklisted dependencies using the logic in extensions_build_system.bzl, the functions in this file get invoked in the individual BUILD files and they get mad if the excluded extension doesn't exist in EXTENSIONS (which should have been WINDOWS_EXTENSIONS in the windows case), and excluding PPC_SKIP_TARGETS and NOBORINGSSL_SKIP_TARGETS` in those cases.

Ideally we would want to construct a unified EXTENSIONS based on blacklists well before evaluating the BUILD files.

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CC: @wrowe @lizan