cmd/go: support -json flag in go version

cmd/go: support -json flag in go version

It supports features described in the issue:

* add -json flag for 'go version -m' to print json encoding of
  runtime/debug.BuildSetting to standard output.
* report an error when specifying -json flag without -m.
* print build settings on seperated line for each binary

Fixes #69712

Change-Id: I79cba2109f80f7459252d197a74959694c4eea1f
Reviewed-on: https://go-review.googlesource.com/c/go/+/619955
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>