test/codegen: document -all_codegen option in README

It is useful to know about the -all_codegen option for running
codegen tests for all platforms. I was puzzling that some codegen
test was not failing on my local machine or on trybot, until I
found this option.

Change-Id: I062cf4d73f6a6c9ebc2258195779d2dab21bc36d
Reviewed-on: https://go-review.googlesource.com/c/go/+/192101Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>

test/codegen/README

@@ -30,6 +30,12 @@ transformation, it can be useful to first run the test harness with a
 toolchain from a released Go version (and verify that the new tests
 fail), and then re-runnig the tests using the devel toolchain.
 
+By default, only checks that are relevant to the current GOOS/GOARCH
+are run. Checks for all platforms can be enabled by specifiying the
+-all_codegen option, as
+
+  $ ../bin/go run run.go -all_codegen codegen
+
 
 - Regexps comments syntax