Simplify handling of references (compressed, full, mixed-static, mixed-dynamic)

[sharon-wang]

Related: ibmruntimes/openj9-openjdk-jdk11#361
More mixed builds discussion: eclipse-openj9/openj9#8878.

Suggested by @keithc-ca in #5657:

I think there should be plans to simplify the handling of references. Back when the only choices were compressed or not, OMR_GC_COMPRESSED_POINTERS was a reasonable flag. We've since added OMR_GC_FULL_POINTERS and OMR_OVERRIDE_COMPRESS_OBJECT_REFERENCES and, in my opinion, it's tough to juggle all the combinations in one's head. I think it would be clearer if there was one macro, say, OMR_GC_POINTER_MODE, which would be configured as one of the following:

* `OMR_GC_POINTER_MODE_FULL` - no compression (the only choice for 32-bit environments)

* `OMR_GC_POINTER_MODE_COMPRESSED` - compressed pointers (what `OMR_GC_COMPRESSED_POINTERS` used to mean)

* `OMR_GC_POINTER_MODE_MIXED_STATIC` - both compressed and non-compressed references supported by compiling certain code twice

* `OMR_GC_POINTER_MODE_MIXED_DYNAMIC` - both compressed and non-compressed references supported via dynamic runtime tests

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Here's a table with all the settings, for reference (based on the implementation in #5657):

Configure Mode	OMR_GC_POINTER_MODE	OMR_GC_COMPRESSED_POINTERS	OMR_GC_FULL_POINTERS	OMR_MIXED_REFERENCES_MODE_STATIC	OMR_OVERRIDE_COMPRESS_OBJECT_REFERENCES
compressed	compressed	ON	OFF	OFF	UNDEFINED
full	full	OFF	ON	OFF	UNDEFINED
mixed-static	mixed	ON	ON	ON	DEFINED (0 or 1)
mixed-dynamic	mixed	ON	ON	OFF	UNDEFINED

Going with Keith's proposal, we would no longer need OMR_MIXED_REFERENCES_MODE_STATIC, since we could work with OMR_GC_POINTER_MODE=OMR_GC_POINTER_MODE_MIXED_STATIC. We could also remove OMR_GC_COMPRESSED_POINTERS/OMR_GC_FULL_POINTERS and replace them with OMR_GC_POINTER_MODE=OMR_GC_POINTER_MODE_FULL/OMR_GC_POINTER_MODE_COMPRESSED. However, we'd still need OMR_OVERRIDE_COMPRESS_OBJECT_REFERENCES so that the separate regular/full libraries built in static can have the appropriate pointer mode set.

[sharon-wang]

FYI @gacholio

[fjeremic]

@amicic @dmitripivkine FYI.