go: fix gcc6 build of bootstrap_go

srcpkgs/go/template

@@ -2,7 +2,7 @@
 pkgname=go
 _bootstrap_version=1.4.2
 version=1.7.1
-revision=2
+revision=3
 wrksrc=go
 hostmakedepends="git ca-certificates"
 short_desc="The Go Programming Language"
@@ -43,6 +43,10 @@ do_extract() {
 
 	tar -xf "${XBPS_SRCDISTDIR}/${pkgname}-${version}/go${version}.src.tar.gz" \
 		-C "${XBPS_BUILDDIR}/go" --strip-components=1
+
+	# Fix a gcc6 warning regarding signed shift left
+	sed -i ${XBPS_BUILDDIR}/go_bootstrap/src/cmd/6c/txt.c \
+		-e's;f->vconst |= (vlong)~0 << 32;f->vconst = (vlong)(~0ull << 32);'
 }
 
 do_build() {