cmd/go: add a 'buildmode' condition for script tests

In CL 208233 I am fixing a panic that occurs only with a specific
build mode. I want that test to run on all platforms that support that
build mode, but the logic for determining support is somewhat
involved.

For now, I am duplicating that logic into the cmd/internal/sys
package, which already reports platform support for other build flags.

We can refactor cmd/go/internal/work to use the extracted function in
a followup CL.

Updates #35759

Change-Id: Ibbaedde4d1e8f683c650beedd10849bc27e7a6e7
Reviewed-on: https://go-review.googlesource.com/c/go/+/208457


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>