misc/cgo/testcshared: avoid writing to GOROOT in tests

The tests in this package invoked 'go install -i -buildmode=c-shared'
in order to generate an archive as well as multiple C header files.

Unfortunately, the behavior of the '-i' flag is inappropriately broad
for this use-case: it not only generates the library and header files
(as desired), but also attempts to install a number of (unnecessary)
archive files for transitive dependencies to
GOROOT/pkg/$GOOS_$GOARCH_testcshared_shared, which may not be writable
— for example, if GOROOT is owned by the root user but the test is
being run by a non-root user.

Instead, for now we generate the header files for transitive dependencies
separately by running 'go tool cgo -exportheader'.

In the future, we should consider how to improve the ergonomics for
generating transitive header files without coupling that to
unnecessary library installation.

Updates #28387
Updates #30316
Updates #35715

Change-Id: I622426a860828020d98f7040636f374e5c766d28
Reviewed-on: https://go-review.googlesource.com/c/go/+/208119


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>