cmd/compile: eagerly CalcStructSize for synthetic ABI types

The next CL is going to replace Type.Width with Type.Size(), but
Type.Size() isn't safe to call concurrently. So this CL calls
CalcStructSize, which *is* allowed to be used concurrently, but then
it's the caller's responsibility to ensure it's called right after
NewStruct.

Change-Id: If9cd81650ccb3a867b4449af757375fa56227901
Reviewed-on: https://go-review.googlesource.com/c/go/+/345483


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>