cgotest attempts to exercise various linkmodes, however with recent refactoring
TestCrossPackageTests is no longer running tests with these linkmodes. Specifying
the linkmode via GOFLAGS restores previous behaviour.

Note that the -ldflags="-linkmode=external -s" case cannot be passed through
as GOFLAGS does not permit spaces in values and -ldflags can only be specified
once.

Fixes #31083.

Change-Id: I2ce6c60da3f3d60495af283ea9122fb68a7a4f41
Reviewed-on: https://go-review.googlesource.com/c/go/+/169779
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>