Apply a slightly different color hammer to arm/release.sh:

- gcc(1) fails to build usr.bin/dtc
 - lack of WITH_GNUCXX=1 causes cc1plus(1) calls to fail
 - u-boot fails to build with clang (hard-coded gcc(1) calls)

Implement the proper incantation of WITH_/WITHOUT_ knobs
to get arm snapshot builds working again.

Since the cc(1) binary is no longer expected to be clang(1),
remove the chroot(8) post-install cc(1) overwrite.

MFC After:	3 days
X-MFC-With:	r264518,r264697,r264698
Tested on:	stable/10@r264677 RPI-B
Sponsored by:	The FreeBSD Foundation

release/arm/release.sh

@@ -92,14 +92,9 @@ install_uboot() {
 }
 
 main() {
-	# Build gcc for use in the chroot for arm builds.
-	# This is not '-j'-safe, so force '-j1' to allow using
-	# additional, non-'-j' options specified in WORLD_FLAGS.
-	eval chroot ${CHROOTDIR} make -C /usr/src/gnu/usr.bin/cc \
-		WITH_GCC=1 ${WORLD_FLAGS} -j1 obj depend all install
 	# Build the 'xdev' target for crochet.
 	eval chroot ${CHROOTDIR} make -C /usr/src \
-		WITHOUT_CLANG_IS_CC=1 WITHOUT_CLANG=1 WITH_GCC=1 \
+		WITH_GCC=1 WITH_GNUCXX=1 WITHOUT_CLANG_IS_CC=1 \
 		XDEV=${XDEV} XDEV_ARCH=${XDEV_ARCH} \
 		${WORLD_FLAGS} xdev