runtime: recreate function called by cgo -gccgo

When using cgo -gccgo calls to C.GoString, C.GoStringN, and C.GoBytes
    are turned into calls to __go_byte_array_to_string and
    __go_string_to_byte_array.  Those functions were removed when the string
    code was copied from Go 1.7, but we still need them for cgo.  While cgo
    should be updated, old versions will exist for some time.
    
    Reviewed-on: https://go-review.googlesource.com/32474

From-SVN: r241743

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-90f12ac1fa72a95e73cb88b6114fa3131c4ca8ee
+069ed35ecbefd2f138ea3132a557ad23a6936a45
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/runtime/string.go

@@ -444,3 +444,20 @@ func gostringw(strw *uint16) string {
 	b[n2] = 0 // for luck
 	return s[:n2]
 }
+
+// These two functions are called by code generated by cgo -gccgo.
+
+//go:linkname __go_byte_array_to_string __go_byte_array_to_string
+func __go_byte_array_to_string(p unsafe.Pointer, l int) string {
+	if l == 0 {
+		return ""
+	}
+	s, c := rawstringtmp(nil, l)
+	memmove(unsafe.Pointer(&c[0]), p, uintptr(l))
+	return s
+}
+
+//go:linkname __go_string_to_byte_array __go_string_to_byte_array
+func __go_string_to_byte_array(s string) []byte {
+	return stringtoslicebyte(nil, s)
+}