Turn on threads support, mainly for ld.gold. In my tests with llvm

this actually worked better than no threads, and threads are turned on
in other packaging distributions.  Have been using this for a while
and it looks like the more tested config upstream.

Reviewed-by: bapt (back in June)

devel/binutils/Makefile

@@ -63,7 +63,8 @@ LDFLAGS+=	-L${LOCALBASE}/lib
 CONFIGURE_ARGS+=	--with-system-zlib \
 			--with-gmp=${LOCALBASE} \
 			--with-mpfr=${LOCALBASE} \
-			--enable-targets=all
+			--enable-targets=all \
+			--enable-threads=yes
 INFO=		as \
 		binutils \
 		gprof \