runtime: allocate mp.cgocallers earlier

Fixes #15061.

Change-Id: I71f69f398d1c5f3a884bbd044786f1a5600d0fae
Reviewed-on: https://go-review.googlesource.com/21398
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/cgocall.go

@@ -113,11 +113,6 @@ func cgocall(fn, arg unsafe.Pointer) int32 {
 	mp.ncgo++
 	defer endcgo(mp)
 
-	// Allocate memory to hold a cgo traceback if the cgo call crashes.
-	if mp.cgoCallers == nil {
-		mp.cgoCallers = new(cgoCallers)
-	}
-
 	// Reset traceback.
 	mp.cgoCallers[0] = 0
 

src/runtime/proc.go

@@ -510,6 +510,11 @@ func mcommoninit(mp *m) {
 	// so we need to publish it safely.
 	atomicstorep(unsafe.Pointer(&allm), unsafe.Pointer(mp))
 	unlock(&sched.lock)
+
+	// Allocate memory to hold a cgo traceback if the cgo call crashes.
+	if iscgo || GOOS == "solaris" || GOOS == "windows" {
+		mp.cgoCallers = new(cgoCallers)
+	}
 }
 
 // Mark gp ready to run.