Makefile: make GOROOT writable before patching

Since go commit 02d24fc25285, the GOROOT directory has been made
read-only, this affects the go1.13beta1 binary tarball as used by Travis
CI. See golang/go#30316

_dev/Makefile

@@ -68,6 +68,8 @@ $(GOROOT_LOCAL)/pkg/.ok_$(VER_OS_ARCH): $(GOROOT_ENV)/pkg | $(GOROOT_LOCAL)/src/
 	cp -r $(GOROOT_ENV)/src $(GOROOT_LOCAL)/
 	cp -r $(GOROOT_ENV)/pkg/include $(GOROOT_LOCAL)/pkg/
 	cp -r $(GOROOT_ENV)/pkg/tool $(GOROOT_LOCAL)/pkg/
+# Ensure GOROOT sources are writable to make patching possible.
+	chmod -R u+w $(GOROOT_LOCAL)/src
 # Apply additional patches to stdlib
 	for p in $(wildcard $(DEV_DIR)/patches/*); do patch -d "$(GOROOT_LOCAL)" -p1 < "$$p"; done
 	touch $@