cmd/compile: use f.Nname.Type() in makeABIWrapper

Apparently, f.Nname.Ntype.Type() doesn't work with types2, as it
doesn't set Ntype, unlike the old type checker. f.Nname.Type()
works for both.

Change-Id: I6fa8a81c9fc7b65e008d9f158b88f0d56d84c3ff
Reviewed-on: https://go-review.googlesource.com/c/go/+/312089


Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>