This makes it much easier to run individual failing subtests.

Use $(go env CC) instead of always defaulting to clang; this makes it
easier to test with other compilers.

Run C binaries to detect incompatible compiler/kernel pairings instead
of sniffing versions.

updates #21196

Change-Id: I0debb3cc4a4244df44b825157ffdc97b5c09338d
Reviewed-on: https://go-review.googlesource.com/52910
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>