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
Add consistency checks to ensure that fragments of a user message
have the same U-bit.

MFC after: 3 days