runtime: rename _defer struct _panic field to panicStack

The gc version of the _defer struct has a _panic field that has a
    completely different meaning. We are going to want that bring that new
    meaning into the gofrontend to improve panic reports with nested
    panic calls. Simplify that by first renaming the existing _panic field.
    
    Reviewed-on: https://go-review.googlesource.com/46454

From-SVN: r249558

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-a81079a81b63714221674f07d13bedc768092f27
+a459f1fdfe0bd365bf2def730e1529052c6487fd
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/runtime/panic.go

@@ -90,13 +90,13 @@ func throwinit() {
 // pfn is a C function pointer.
 // arg is a value to pass to pfn.
 func deferproc(frame *bool, pfn uintptr, arg unsafe.Pointer) {
-	n := newdefer()
-	n.frame = frame
-	n._panic = getg()._panic
-	n.pfn = pfn
-	n.arg = arg
-	n.retaddr = 0
-	n.makefunccanrecover = false
+	d := newdefer()
+	d.frame = frame
+	d.panicStack = getg()._panic
+	d.pfn = pfn
+	d.arg = arg
+	d.retaddr = 0
+	d.makefunccanrecover = false
 }
 
 // Allocate a Defer, usually using per-P pool.
@@ -502,7 +502,7 @@ func currentDefer() *_defer {
 	// the panic stack. We do not want to recover it if that panic
 	// was on the top of the panic stack when this function was
 	// deferred.
-	if d._panic == gp._panic {
+	if d.panicStack == gp._panic {
 		return nil
 	}
 
@@ -731,7 +731,7 @@ func gorecover() interface{} {
 // function like recover.
 func deferredrecover() interface{} {
 	gp := getg()
-	if gp._defer == nil || gp._defer._panic != gp._panic {
+	if gp._defer == nil || gp._defer.panicStack != gp._panic {
 		return nil
 	}
 	return gorecover()

libgo/go/runtime/runtime2.go

@@ -698,7 +698,7 @@ type _defer struct {
 	// deferred.  This function can not recover this value from
 	// the panic stack.  This can happen if a deferred function
 	// has a defer statement itself.
-	_panic *_panic
+	panicStack *_panic
 
 	// The function to call.
 	pfn uintptr