cmd/go: fix cgo test when min macOS version is set

Regression tests for #24161 use a macro to conditionally compile some
stub definitions. The macro tests that the minimum macOS version is
less than 10.12.

We get duplicate definitions when building this test with
CGO_CFLAGS=-mmacosx-version-min=10.x where 10.x < 10.12. With this
change, we use a different macro, __MAC_OS_X_VERSION_MAX_ALLOWED__,
which tests the SDK version instead of the minimum macOS version. This
checks whether these definitions are present in headers.

After this change, 'go tool dist test cgo_test' should pass with
CGO_FLAGS=-mmacosx-version-min=10.10.

Updates #35459

Change-Id: I88d63601c94b0369c73c38d216a2d41ba7d4e579
Reviewed-on: https://go-review.googlesource.com/c/go/+/216243


Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>