runtime: mark race instrumentation callbacks as nosplitstack

It speedups the race detector somewhat, but also prevents
getcallerpc() from obtaining lessstack().

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/6812091

src/pkg/runtime/race.c

@@ -47,6 +47,8 @@ runtime·racefini(void)
 }
 
 // Called from instrumented code.
+// If we split stack, getcallerpc() can return runtime·lessstack().
+#pragma textflag 7
 void
 runtime·racewrite(uintptr addr)
 {
@@ -58,6 +60,8 @@ runtime·racewrite(uintptr addr)
 }
 
 // Called from instrumented code.
+// If we split stack, getcallerpc() can return runtime·lessstack().
+#pragma textflag 7
 void
 runtime·raceread(uintptr addr)
 {
@@ -69,6 +73,7 @@ runtime·raceread(uintptr addr)
 }
 
 // Called from instrumented code.
+#pragma textflag 7
 void
 runtime·racefuncenter(uintptr pc)
 {
@@ -83,6 +88,7 @@ runtime·racefuncenter(uintptr pc)
 }
 
 // Called from instrumented code.
+#pragma textflag 7
 void
 runtime·racefuncexit(void)
 {