[NFC] Refactor byteswapped writes

This is an extremely small refactoring to writing byte-swapped values
in the PSV data. I've broken the PPC-BE bots a few times with changes
here because of the fragility of how byte-swapping was being done. This
should make it less likely for me to break BE builders in the future.

Reviewed By: bogner, bob80905

Differential Revision: https://reviews.llvm.org/D156814