misc/cgo/testsanitizers: don't run some TSAN tests on GCC < 7

Before GCC 7 defined __SANITIZE_THREAD__ when using TSAN,
runtime/cgo/libcgo.h could not determine reliably whether TSAN was in
use when using GCC.

Fixes #15983.

Change-Id: I5581c9f88e1cde1974c280008b2230fe5e971f44
Reviewed-on: https://go-review.googlesource.com/23833
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Hudson-Doyle <michael.hudson@canonical.com>

misc/cgo/testsanitizers/test.bash

@@ -135,11 +135,28 @@ if test "$tsan" = "yes"; then
     testtsan tsan3.go
     testtsan tsan4.go
 
+    # These tests are only reliable using clang or GCC version 7 or later.
+    # Otherwise runtime/cgo/libcgo.h can't tell whether TSAN is in use.
+    ok=false
+    if ${CC} --version | grep clang >/dev/null 2>&1; then
+	ok=true
+    else
+	ver=$($CC -dumpversion)
+	major=$(echo $ver | sed -e 's/\([0-9]*\).*/\1/')
+	if test "$major" -lt 7; then
+	    echo "skipping remaining TSAN tests: GCC version $major (older than 7)"
+	else
+	    ok=true
+	fi
+    fi
+
+    if test "$ok" = "true"; then
 	# This test requires rebuilding os/user with -fsanitize=thread.
 	testtsan tsan5.go "CGO_CFLAGS=-fsanitize=thread CGO_LDFLAGS=-fsanitize=thread" "-installsuffix=tsan"
 
 	# This test requires rebuilding runtime/cgo with -fsanitize=thread.
 	testtsan tsan6.go "CGO_CFLAGS=-fsanitize=thread CGO_LDFLAGS=-fsanitize=thread" "-installsuffix=tsan"
+    fi
 fi
 
 exit $status