This change removes the build-time check that GopherJS was built
with Go version 1.12, and replaces it with a runtime check instead.

Add and document a new GopherJS-specific environment variable:

GOPHERJS_GOROOT - if set, GopherJS uses this value as the default GOROOT value,
                  instead of using the system GOROOT as the default GOROOT value

The main reason that GopherJS required Go 1.12 to be built in the
past was as a way of ensuring that the Go 1.12 standard library
code is available on disk, which GopherJS 1.12-2 needs to build
Go code. Now that the Go distribution can provided to GopherJS
with convenience via the GOPHERJS_GOROOT environment variable,
we can allow GopherJS to be built with a newer version of Go,
as long as a Go 1.12 distribution is provided via GOPHERJS_GOROOT.

Update GopherJS installation instructions to support being installed
with Go 1.12 and newer versions of Go.

The Go distribution version check is added to build.NewSession.
That check may fail, and so it was necessary to add an error return
value to the signature of build.NewSession. The build API is deemed
semi-internal, so a breaking API change is unfortunate but acceptable.

Regenerate ./compiler/natives with:

	go generate ./...

Fixes #941.

GitHub-Pull-Request: #966