[libomptarget][nfc] Replace static const with enum

[libomptarget][nfc] Replace static const with enum

Semantically identical. Replaces 0xff... with ~0 to spare counting the f.
Has the advantage that the compiler doesn't need to prove the 4/8 byte
value dead before discarding it, and sidesteps the compilation question
associated with what static means for a single source language.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D93328