cmd/compile: increment iexportVersionGenerics

[findleyr]

iexportVersionGenerics was left at 1 to facilitate incrementing transition to export data version 2.

If we're going to increment it, we should do so before the beta. In a team discussion today, we decided that we should increment it.

CC @danscales @heschi @mdempsky @griesemer

[gopherbot]

Change https://golang.org/cl/367634 mentions this issue: cmd/compile: change iexportVersionGeneric to 2