Use FeatureConfiguration to compute value of CC_FLAGS make variable

This cl introduces new action_config type for Crosstool named 'generic'. This
can be used to set the value of CC_FLAGS make variable using much more
expressive mechanism (action_configs + features) than previous make_variable
Crosstool messages. This has been requested by the C++ LPT.

However, as FeatureConfiguration needs RuleContext, CC_FLAGS cannot be
computed using configuration only anymore. Also, FeatureConfiguration is C++
rules specific class, and Bazel build-base cannot depend on it. Therefore we
cannot use FeatureConfiguration for ExtraActions, for example. Because it cannot
be made perfect, this cl is not updating all the possible places that expand
make variables but limits the scope to:

* genrule (the only widely used rule that often expands make variables)
* *_test (CC_FLAGS is used by Tensorflow in the 'args' attribute)
* cc_rules (people sometimes set 'copts' to something like:
  "-DCC_STRING=\\\"$(CC)\\\" -DCC_FLAGS_STRING=\"$(CC_FLAGS)\""

The long term plan is to use Skylark C++ API to get C++ command lines, so
CC_FLAGS together with this inconsistent solution will be removed.

RELNOTES: CC_FLAGS can be defined using 'generic' action_config in CROSSTOOL
PiperOrigin-RevId: 157202883

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