Fix macros for converting little endian to host for TF_TSRT_OFFSET GetSize

Make the macro that converts little endian data do nothing on little endian hosts,
and byte swap otherwise.
This only affects getting the size of TStrings of type "Offset".

Added a test for TStrings of type "Offset" that checks if type and size are consistent.

PiperOrigin-RevId: 400789721
Change-Id: I1398bffd842ab1631614b212b7c3a2af88d99538