cmd/go: support -ccflags in 'go test'

Fixes #7810.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/89050043

src/cmd/go/testflag.go

@@ -76,6 +76,7 @@ var testFlagDefn = []*testFlagSpec{
 	{name: "p"},
 	{name: "x", boolVar: &buildX},
 	{name: "work", boolVar: &buildWork},
+	{name: "ccflags"},
 	{name: "gcflags"},
 	{name: "exec"},
 	{name: "ldflags"},