This is required to properly unwind reflect.methodValueCall/makeFuncStub.
Fixes #5954.
Stats for 'go install std':
61849 total INSTCALL
24655 currently have ArgSize metadata
27278 have ArgSize metadata with this change
godoc size before: 11351888, after: 11364288

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/12163043