libgo: update to Go 1.8 release candidate 1

Compiler changes:
      * Change map assignment to use mapassign and assign value directly.
      * Change string iteration to use decoderune, faster for ASCII strings.
      * Change makeslice to take int, and use makeslice64 for larger values.
      * Add new noverflow field to hmap struct used for maps.
    
    Unresolved problems, to be fixed later:
      * Commented out test in go/types/sizes_test.go that doesn't compile.
      * Commented out reflect.TestStructOf test for padding after zero-sized field.
    
    Reviewed-on: https://go-review.googlesource.com/35231

gotools/:
	Updates for Go 1.8rc1.
	* Makefile.am (go_cmd_go_files): Add bug.go.
	(s-zdefaultcc): Write defaultPkgConfig.
	* Makefile.in: Rebuild.

From-SVN: r244456

libgo/go/runtime/mstkbar.go

@@ -150,6 +150,10 @@ var firstStackBarrierOffset = 1024
 // gcMaxStackBarriers returns the maximum number of stack barriers
 // that can be installed in a stack of stackSize bytes.
 func gcMaxStackBarriers(stackSize int) (n int) {
+	if debug.gcstackbarrieroff > 0 {
+		return 0
+	}
+
 	if firstStackBarrierOffset == 0 {
 		// Special debugging case for inserting stack barriers
 		// at every frame. Steal half of the stack for the