make.bat: do not exit builder on successful completion of make.bat (fixes build)

LGTM=dvyukov
R=golang-codereviews, dvyukov
CC=golang-codereviews
https://golang.org/cl/118020043

src/make.bat

@@ -115,6 +115,6 @@ goto end
 
 :fail
 set GOBUILDFAIL=1
+if x%GOBUILDEXIT%==x1 exit %GOBUILDFAIL%
 
 :end
-if x%GOBUILDEXIT%==x1 exit %GOBUILDFAIL%