runtime: convert timers to Go

LGTM=rsc
R=golang-codereviews, ruiu, rsc, daniel.morsing
CC=golang-codereviews, khr
https://golang.org/cl/123700044
diff --git a/src/pkg/runtime/thunk.s b/src/pkg/runtime/thunk.s
index b54d9ed..1f83438 100644
--- a/src/pkg/runtime/thunk.s
+++ b/src/pkg/runtime/thunk.s
@@ -11,6 +11,18 @@
 #define JMP B
 #endif
 
+TEXT time·runtimeNano(SB),NOSPLIT,$0-0
+	JMP     runtime·gonanotime(SB)
+
+TEXT time·Sleep(SB),NOSPLIT,$0-0
+	JMP     runtime·timeSleep(SB)
+
+TEXT time·startTimer(SB),NOSPLIT,$0-0
+	JMP     runtime·startTimer(SB)
+
+TEXT time·stopTimer(SB),NOSPLIT,$0-0
+	JMP     runtime·stopTimer(SB)
+
 TEXT sync·runtime_Syncsemacquire(SB),NOSPLIT,$0-0
 	JMP	runtime·syncsemacquire(SB)