Create single configure option for reference representation

[sharon-wang]

To avoid having to consider the precedence of --with-mixedrefs (introduced in #359) and --with-noncompressedrefs, we should introduce a new configure option --with-references=[full|compressed|mixed-static|mixed-dynamic] to cover all reference representation modes.

Proposed by @keithc-ca (#359 (review)).

These changes will need to go in each of the extensions repos.

More mixed builds discussion: eclipse-openj9/openj9#8878

[keithc-ca]

I'm not thrilled with the names of the choices, but the list is a start and we appear to have agreement on the meanings of the choices.

Note, this will have implications for adoptopenjdk.net (i.e. 'xl' builds using --with-noncompressedrefs).

[sharon-wang]

When we address #361 we should add more checking (e.g. mixedrefs should not be accepted for (supported) 32-bit targets).

From ibmruntimes/openj9-openjdk-jdk8#459