Add a SYSDECODE_ABI_ prefix to the ABI enums to avoid potential collisions.

[jhb]

Summary

Add a SYSDECODE_ABI_ prefix to the ABI enums to avoid potential collisions.

If this was a C++ library I'd stick everything in a 'sysdecode::' namespace
and call it a day, but this is C.

Suggested by: jmallett

Test Plan

• tinderbox built ok