MFC r263373, r268402

MFC r263373, r268402

  Add a way to apply CFLAGS only when building the given architecture.  This
  is useful primarily on a system used for cross-building, when you have a
  set of flags to apply to the TARGET_ARCH being cross-built but don't want
  those settings applied to building the cross-tools or other components that
  run on the build host machine.

  Support CXXFLAGS.${MACHINE_ARCH} as well as CFLAGS.  This allows different
  C++ options for toolchain versus target when cross-building.

git-svn-id: svn://svn.freebsd.org/base/stable/10@269102 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f