[v3] libstdc++/21769: add add_options_for_no_pch and use it in c99_classification_macros_c.cc

[Paolo Carlini <paolo dot carlini at oracle dot com>]

Hi,

I decided to go ahead for now with the simple solution suggested by
Janis, which allows to clean up the spurious xpass of
c99_classification_macros_c.cc with PCHs which we have been seeing for
way too much time.

Port maintainers may want to go through again the various dg-*
directives and make sure still suit their needs: the important point is
that, normally, that is without any spurious effect of PCHs, the
testcase is *expected to fail* given the current libstdc++
implementation (at some point in the future we may want to reconsider
whether this is really a failure, not now)

Thanks,
Paolo.

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