Fix LinkFieldsComparator.

Define order for primitive types with the same sizes.
Previously, the comparator would consider the fields equal
so the order would depend on std::sort() implementation.
Changing the STL implementation could silently change the
field offsets. (And, unlike std::stable_sort(), the
std::sort() doesn't even need to be deterministic.)

Change-Id: I91fa562f82447606aced64643bea8c70784766b5