MFC: r263881, r263952, r263983, r264068, r264090, r264131

MFC: r263881, r263952, r263983, r264068, r264090, r264131

Some xdev fixes:

- if TARGET_ARCH is not defined and XDEV_ARCH is defined then early define
  TARGET_ARCH to the valud of XDEV_ARCH: This allow the xdev-build target
  to be able to correctly chose the compiler it needs to build
- Allow overwriting XDTP to allow a user to not chose where the xdev env will
  live in
- Fix build for gcc only xdev (like ia64) by providing the proper -B to the
  toolchain and not relying on gcc being installed already in base
- Fix TOOLS_PREFIX so the generated toolchain has the right default sysroot when
  installed intead of getting the DESTDIR one
- Fix supporting DESTDIR
- Also overwrite CXX (needed for cross building c++ libraries with clang) and
  CPP (needed to cross build some libraries when gcc is the target default
  compiler but gcc is not installed on the building host)
- Prevent XDTP from being a relative path:
  XDTP is used as the default SYSROOT for clang and thus should be an absolute path.
- Respect default CC

git-svn-id: svn://svn.freebsd.org/base/stable/10@264372 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f