MFC r264518, r264697, r264698, r264703:

r264518:
  Fix armv6 build failure: "armv6-freebsd-gcc: not found"

r264697:
  When building the 'xdev' target when WITH_GCC=1 and
  WITHOUT_CLANG_IS_CC=1, it appears WITHOUT_CLANG=1 must
  also be set.

  While here, reorder the lines to put the WITH_* and
  WITHOUT_* entries on the same line.

r264698:
  Fix EOL escape.

r264703:
  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 no longer needs to be cc(1) instead of
  clang(1), remove the chroot(8) post-install cc(1) overwrite.

Sponsored by:	The FreeBSD Foundation