[release-branch.go1] runtime: handle and test large map values

[release-branch.go1] runtime: handle and test large map values

««« backport 98488e2e38ee
runtime: handle and test large map values

This is from CL 5451105 but was dropped from that CL.
See also CL 6137051.

The only change compared to 5451105 is to check for
h != nil in reflect·mapiterinit; allowing use of nil maps
must have happened after that original CL.

Fixes #3573.

R=golang-dev, dave, r
CC=golang-dev
https://golang.org/cl/6215078

»»»