runtime: avoid write barriers when calling deferred function

Calling a deferred function currently requires changing from a uintptr
    to the function code to a Go function value. That is done by setting
    the value of a func local variable using unsafe.Pointer. The local
    variable will always be on the stack. Adjust the code that sets the
    local variable to avoid generating a write barrier.
    
    A write barrier is never needed here. Also, for deferreturn, we must
    avoid write barriers entirely when called from a cgo function; that
    requires more than just this, but this is a start.
    
    The test for this is runtime tests that use the go tool; these are not
    currently run, but they will be in the future.
    
    Reviewed-on: https://go-review.googlesource.com/46455

From-SVN: r249559

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-a459f1fdfe0bd365bf2def730e1529052c6487fd
+73b14da15ec731837ce2a45db658142bfbf5fe22
 
 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

@@ -194,7 +194,7 @@ func deferreturn(frame *bool) {
 			// The gc compiler does this using assembler
 			// code in jmpdefer.
 			var fn func(unsafe.Pointer)
-			*(**uintptr)(unsafe.Pointer(&fn)) = &pfn
+			*(*uintptr)(unsafe.Pointer(&fn)) = uintptr(unsafe.Pointer(&pfn))
 			fn(d.arg)
 		}
 
@@ -259,7 +259,7 @@ func checkdefer(frame *bool) {
 			gp._defer = d.link
 
 			var fn func(unsafe.Pointer)
-			*(**uintptr)(unsafe.Pointer(&fn)) = &pfn
+			*(*uintptr)(unsafe.Pointer(&fn)) = uintptr(unsafe.Pointer(&pfn))
 			fn(d.arg)
 
 			freedefer(d)
@@ -345,7 +345,7 @@ func Goexit() {
 
 		if pfn != 0 {
 			var fn func(unsafe.Pointer)
-			*(**uintptr)(unsafe.Pointer(&fn)) = &pfn
+			*(*uintptr)(unsafe.Pointer(&fn)) = uintptr(unsafe.Pointer(&pfn))
 			fn(d.arg)
 		}
 
@@ -446,7 +446,7 @@ func gopanic(e interface{}) {
 
 		if pfn != 0 {
 			var fn func(unsafe.Pointer)
-			*(**uintptr)(unsafe.Pointer(&fn)) = &pfn
+			*(*uintptr)(unsafe.Pointer(&fn)) = uintptr(unsafe.Pointer(&pfn))
 			fn(d.arg)
 
 			if p.recovered {