lld was using RISCVISAInfo(unsigned XLen,
RISCVISAUtils::OrderedExtensionMap &Exts). This required a call to
RISCVISAInfo::postProcessAndChecking to validate the RISCVISAInfo that
was created. This exposes too much about RISCVISAInfo to lld.
Replace with a new RISCVISAInfo::createFromExtMap that is responsible
for creating the object and calling postProcessAndChecking.