compiletest does not support taking the disjunction/union of multiple "only" directives

Say I want to run a particular test (or a particular revision) on all x86 targets -- 32bit and 64bit. Currently, there's no good way to do that: only-x86 and only-x86_64 select one of these cases, respectively, but I can't say "either one". If I use multiple only annotations, that takes the intersection, i.e. they all have to apply -- which makes sense as semantics, but is not helpful here. So instead what I usually do is have a separate 32bit and 64bit revision, and make them identical, but that can have significant overhead -- in particular for FileCheck tests where one cannot say "this annotation is active in the following two revisions".

I think the biggest problem here is coming up with a good syntax.^^ Ideally, we'd have a concept of an "only annotation", so e.g. instead of only-x86 we'd use something like only: x86 . Then it would be easy to say that only: a b c means that at least one of the conditions a, b, c must be satisfied.

Cc @jieyouxu
(not sure whom else to ping for compiletest things)

Start by locating compiletest's handling of the existing only-x86 and only-x86_64 directives. Review how multiple only annotations are parsed and combined, then define and implement syntax for disjunctive conditions. Done means one test or revision can target either 32-bit or 64-bit x86 without duplicating revisions, with coverage for the new behavior.