test: re-enable AsmCheck tests for types2-based frontends

run.go has logic for being able to run tests with various -G flags,
but not all test types (in particular, "asmcheck" tests) support
configuring non-default -G levels. The intention was that these tests
would continue running in the default mode (at the time -G=0), but at
some point it seems like we accidentally disabled them all
together (if it ever worked correctly in the first place).

Fixes #48247.

Change-Id: I13917cb0012cbe522d29b23b888de6136872ead4
Reviewed-on: https://go-review.googlesource.com/c/go/+/348671


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Munday <mike.munday@lowrisc.org>