go: fix bootstrap with gcc7

srcpkgs/go/template

@@ -53,6 +53,12 @@ do_extract() {
 		-e's;f->vconst |= (vlong)~0 << 32;f->vconst = (vlong)(~0ull << 32);'
 	sed -i ${XBPS_BUILDDIR}/go_bootstrap/src/cmd/8g/ggen.c \
 		-e's;-1LL<<(t->width\*8-1);~0ULL<<(t->width*8-1);'
+
+	# Disable gcc7 fallthrough warnings
+	sed -i ${XBPS_BUILDDIR}/go_bootstrap/src/make.bash \
+		-e's;-Werror;-Werror -Wimplicit-fallthrough=0;'
+	sed -i ${XBPS_BUILDDIR}/go_bootstrap/src/cmd/dist/build.c \
+		-e's;"-Werror",;"-Werror", "-Wimplicit-fallthrough=0",;'
 }
 
 do_build() {