runtime: introduce GODEBUG env var
Currently it replaces GOGCTRACE env var (GODEBUG=gctrace=1).
The plan is to extend it with other type of debug tracing,
e.g. GODEBUG=gctrace=1,schedtrace=100.

R=rsc
CC=bradfitz, daniel.morsing, gobot, golang-dev
https://golang.org/cl/10026045
diff --git a/src/pkg/runtime/crash_test.go b/src/pkg/runtime/crash_test.go
index 929d4a9..af19274 100644
--- a/src/pkg/runtime/crash_test.go
+++ b/src/pkg/runtime/crash_test.go
@@ -14,7 +14,7 @@
 	"text/template"
 )
 
-// testEnv excludes GOGCTRACE from the environment
+// testEnv excludes GODEBUG from the environment
 // to prevent its output from breaking tests that
 // are trying to parse other command output.
 func testEnv(cmd *exec.Cmd) *exec.Cmd {
@@ -22,7 +22,7 @@
 		panic("environment already set")
 	}
 	for _, env := range os.Environ() {
-		if strings.HasPrefix(env, "GOGCTRACE=") {
+		if strings.HasPrefix(env, "GODEBUG=") {
 			continue
 		}
 		cmd.Env = append(cmd.Env, env)