go/internal/gcimporter,cmd/compile/internal/importer: use testenv.MustHaveGoBuild...

go/internal/gcimporter,cmd/compile/internal/importer: use testenv.MustHaveGoBuild directly

These tests previously had a “skipSpecialPlatforms” function, added in
CL 8611 to skip tests on (apparently) NaCL and iOS. The iOS builders
no longer match the condition (GOOS=ios is its own thing now), and the
NaCL port no longer exists.

The name of the function also isn't very evocative, since it doesn't
say what is “special” about the platforms to cause them to be
skipped.m

Since the check is intending to run the tests only on platforms where
gc export data is available, and to a first approximation
“gc export data is available” on exactly the platforms that can run
“go build” to produce that export data, we can use the testenv function
instead of a one-off.

Updates #38485.

Change-Id: I8f38b9604300d165147f8942e945ab762419fad7
Reviewed-on: https://go-review.googlesource.com/c/go/+/444155
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>