runtime: restore scavenger constants

Once and for all.
Broken in cl/108640043.
I've messed it before. To test scavenger-related changes
one needs to alter the constants during final testing.
And then it's very easy to submit with the altered constants.

LGTM=rsc
R=golang-codereviews
CC=golang-codereviews, rsc
https://golang.org/cl/136720044

src/pkg/runtime/extern.go

@@ -46,6 +46,8 @@ a comma-separated list of name=val pairs. Supported names are:
 	schedtrace: setting schedtrace=X causes the scheduler to emit a single line to standard
 	error every X milliseconds, summarizing the scheduler state.
 
+	scavenge: scavenge=1 enables debugging mode of heap scavenger.
+
 The GOMAXPROCS variable limits the number of operating system threads that
 can execute user-level Go code simultaneously. There is no limit to the number of threads
 that can be blocked in system calls on behalf of Go code; those do not count against

src/pkg/runtime/proc.c

@@ -2632,10 +2632,15 @@ sysmon(void)
 	G *gp;
 
 	// If we go two minutes without a garbage collection, force one to run.
-	forcegcperiod = 2*60*1e6;
+	forcegcperiod = 2*60*1e9;
 	// If a heap span goes unused for 5 minutes after a garbage collection,
 	// we hand it back to the operating system.
-	scavengelimit = 5*60*1e6;
+	scavengelimit = 5*60*1e9;
+	if(runtime·debug.scavenge > 0) {
+		// Scavenge-a-lot for testing.
+		forcegcperiod = 10*1e6;
+		scavengelimit = 20*1e6;
+	}
 	lastscavenge = runtime·nanotime();
 	nscavenge = 0;
 	// Make wake-up period small enough for the sampling to be correct.

src/pkg/runtime/runtime.c

@@ -302,6 +302,7 @@ runtime·tickspersecond(void)
 	return res;
 }
 
+#pragma dataflag NOPTR
 DebugVars	runtime·debug;
 
 static struct {
@@ -314,6 +315,7 @@ static struct {
 	{"gcdead", &runtime·debug.gcdead},
 	{"scheddetail", &runtime·debug.scheddetail},
 	{"schedtrace", &runtime·debug.schedtrace},
+	{"scavenge", &runtime·debug.scavenge},
 };
 
 void

src/pkg/runtime/runtime.h

@@ -562,6 +562,7 @@ struct DebugVars
 	int32	gcdead;
 	int32	scheddetail;
 	int32	schedtrace;
+	int32	scavenge;
 };
 
 extern bool runtime·precisestack;