cmd/compile: set the proper export version number before Go 1.18

[danscales]

When we get closer to the beta for 1.18, we need to set the export version number to its new value,
iexportVersionGo1_18 = 2.

We are temporarily setting the export version back to 1 (iexportVersionGoPosCol, also iexportVersionGoGenerics, since generic functions/types are backward compatible) for ease of internal testing, especially given a lot of x/tools and third-party tools that will break on the new export version.

[danscales]

https://go-review.googlesource.com/c/go/+/341211 is the change the needs to be mostly reverted to fix this issue.

[gopherbot]

Change https://golang.org/cl/341211 mentions this issue: [dev.typeparams] cmd/compile: change export version to 1.17 for testing

[gopherbot]

Change https://golang.org/cl/357049 mentions this issue: cmd/compile: update the export version for generics

[danscales]

@findleyr Is there anything more on your side about the export version that you need to do? If so, I can assign this bug to you as a reminder. Otherwise, I think we can just close this issue, right?

[findleyr]

Yes, I think I still need to clean up once this has had enough time to propagate. Reassigning to myself, but also removing the release-blocker label.