cmd/dist: enable more cgo tests if GO_GCFLAGS != ""
Currently, we have several tests disabled if GO_GCFLAGS is non-empty.
Long ago, this was critical because many of these tests use "go
install" with no -gcflags and would thus overwrite std packages in
GOROOT built with -gcflags=$GO_GCFLAGS. Now these packages all live in
the build cache, so this is no longer a concern.
The other reason for this (the reason given in the code comment), is
that these tests will rebuild significant portions of std without
flags. While this is still theoretically true, there are many tests
that run "go build" with no -gcflags, so these tests don't contribute
much overall.
Empirically, on my linux/amd64 host, running these tests at all grows
the Go build cache by 14%, from 1.899 GB to 2.165 GB. When building
with GO_GCFLAGS="-N -l" (the only use case on the builders), enabling
them grows the Go build cache by 18%, from 1.424 GB to 1.684 GB. This
is only a 4 percentage point difference, and still results in a build
cache that's smaller than the default build
Given all this, there's little reason to carry the complexity of
disabling these tests when GO_GCFLAGS != "". Removing this condition
is a step toward running these as regular cmd tests.