[RISCV] Fix collectNonISAExtFeature returning negative extension features (#76962)

collectNonISAExtFeature was returning any negative extension features,
e.g.
given an input of

+zifencei,+m,+a,+save-restore,-zbb,-relax,-zfa

It would return

+save-restore,-zbb,-relax,-zfa

Because negative extensions aren't emitted when calling
toFeatureVector(), and
so were considered missing. Hence why we still see "-zfa" and "-zfb" in
the tests for
the full arch string attributes, even though with a full arch string we
should be overriding the extensions.

This fixes it by using RISCVISAInfo::isSupportedExtensionFeature instead
to
check if a feature is an ISA extension.