Expose the ILP32/LP64 programming environments based on
ILP32__/LP64__ instead of by architecture.

The list was incomplete (previous commits purged invalid architectures,
like alpha, but failed to add new ones). It's best to base the symbol
presence on whether or not the architecture is ILP32 / LP64 capable, per
the compiler.

This fixes the ILP32/LP64 program environments on some architectures like
arm64, and by proxy fixes the tests on those architectures.

MFC after: 1 month
Reviewed by: no one (timed out on feedback from imp)
Differential Revision: D10787