Play musical-KNOBS a bit more with release/arm/release.sh.

For stable/10, r264703 sets the correct WITH/WITHOUT
knobs to get xdev built with the arm-freebsd-gcc binary
installed.  Unfortunately, the same fix does not work on
head/.

Also, quite to my amazement, WITH_GCC=1 and WITH_GNUCXX=1
causes xdev to fail spectactularly at least on r264791.

The situation as it stands is:

 - gcc(1) is needed for the u-boot build.
 - cc(1) *cannot* be clang(1)

To shoe-horn the toolchain to make 'xdev' give what is
needed, remove WITH_GNUCXX=1 and add WITH_GCC_BOOTSTRAP=1.

MFC After:	1 week
X-MFC-To:	stable/10 only
X-MFC-Note:	after stable/10 is broken in this way...
Sponsored by:	The FreeBSD Foundation

release/arm/release.sh

@@ -94,7 +94,7 @@ install_uboot() {
 main() {
 	# Build the 'xdev' target for crochet.
 	eval chroot ${CHROOTDIR} make -C /usr/src \
-		WITH_GCC=1 WITH_GNUCXX=1 WITHOUT_CLANG_IS_CC=1 \
+		WITH_GCC=1 WITH_GCC_BOOTSTRAP=1 WITHOUT_CLANG_IS_CC=1 \
 		XDEV=${XDEV} XDEV_ARCH=${XDEV_ARCH} \
 		${WORLD_FLAGS} xdev