[TableGen] Stabilize sort in GET_SUBTARGETINFO_MACRO block

The sort of the elements in the GET_SUBTARGETINFO_MACRO block is done on
the "Name" field of each record. This field is not guaranteed to be unique,
is not guaranteed to even have a value at all, and is not used in the
output anyway. Change to sort on the "FieldName" field which should be
unique.

Problem spotted when lib/Target/PowerPC/PPCGenSubtargetInfo.inc changed
unexpectedly.

Differential Revision: https://reviews.llvm.org/D153371