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/math/jn.go

@@ -174,7 +174,7 @@ func Jn(n int, x float64) float64 {
 			q1 := w*z - 1
 			k := 1
 			for q1 < 1e9 {
-				k += 1
+				k++
 				z += h
 				q0, q1 = q1, z*q1-q0
 			}