test/codegen: mention -all_codegen in the README

For performance reasons (avoiding costly cross-compilations) CL 177577
changed the codegen test harness to only run the tests for the
machine's GOARCH by default.

This change updates the codegen README accordingly, explaining what
all.bash does run by default and how to perform the tests for all
architectures.

Fixes #33924

Change-Id: I43328d878c3e449ebfda46f7e69963a44a511d40
Reviewed-on: https://go-review.googlesource.com/c/go/+/192619


Reviewed-by: Daniel Martí <mvdan@mvdan.cc>