lib/Target/SubtargetFeature.cpp asserts that the FeatureKV[] table be sorted

by its first field, but TableGen doesn't actually enforce creating it that 
way. TableGen sorts the records that will be used to create it by the names 
of the records, not the Name field of those records.

This patch corrects the sort to use the "Name" field of the record as the 
sort key.

llvm-svn: 56106