cmd/compile: include PGO devirtualization targets in export data

[prattmic]

When performing PGO devirtualization, we will look for callees reported by the profile from export data. If they are present, great! If not, it may be because that callee doesn't exist anymore, or it may exist but simply wasn't included in export data because it wasn't otherwise needed in export data.

For PGO inlining, we explicitly include bodies of functions that we expect to be PGO inlining targets. We could do the same with devirtualization, explicitly including any function in export data if we expect it to be a devirtualization callee.

I discussed this with @mdempsky a few months ago, who suggested that in addition to (or instead of?) IR-level PGO annotation, we also do analysis at the cmd/compile/internal/syntax level so that unified export generation could use PGO information to decide what needs to be exported.

cc @cherrymui @aclements