[RISCV] Deduplicate RISCVISAInfo::toFeatures/toFeatureVector. NFC (#76942)

toFeatures and toFeatureVector both output a list of target feature
flags, just with a slightly different interface. toFeatures keeps any
unsupported extensions, and also provides a way to append negative
extensions (AddAllExtensions=true).

This patch combines them into one function, so that a later patch will
be be able to get a std::vector of features that includes all the
negative extensions, which was previously only possible through the
StrAlloc interface.