common/build-style: add void-cross

this introduces a new build-style void-cross, which can be used
to write system crosstoolchain templates; this is to reduce the
amount of maintenance, resolve existing problems with the cross
toolchain templates and remove repeated code

common/environment/build-style/void-cross.sh

@@ -0,0 +1,13 @@
+lib32disabled=yes
+nopie=yes
+create_wrksrc=yes
+
+nostrip_files+=" libcaf_single.a libgcc.a libgcov.a libgcc_eh.a
+ libgnarl_pic.a libgnarl.a libgnat_pic.a libgnat.a libgmem.a"
+
+# glibc crosstoolchains not available on musl hosts yet
+if [ -z "$archs" -a "${cross_triplet/-musl}" = "${cross_triplet}" ]; then
+	if [ "$XBPS_TARGET_LIBC" != "glibc" ]; then
+		archs="~*-musl"
+	fi
+fi