runtime: set startpc field when starting a new goroutine

This puts the right value in a trace--previously it was always zero.

Change-Id: I51e31ea8e6d9baea3806465716f4b0dedb7393f1
Reviewed-on: https://go-review.googlesource.com/41139
Reviewed-by: Than McIntosh <thanm@google.com>
diff --git a/libgo/go/runtime/proc.go b/libgo/go/runtime/proc.go
index 28fbe5b..eb69140 100644
--- a/libgo/go/runtime/proc.go
+++ b/libgo/go/runtime/proc.go
@@ -2566,6 +2566,7 @@
 
 	newg.param = arg
 	newg.gopc = getcallerpc(unsafe.Pointer(&fn))
+	newg.startpc = fn
 	if isSystemGoroutine(newg) {
 		atomic.Xadd(&sched.ngsys, +1)
 	}