Fix GCC 4.2.1 to honor --sysroot for includes.

Fix GCC 4.2.1 to honor --sysroot for includes.

- Change the C++ directory entries to honor --sysroot if it is set.
- Don't define CROSS_INCLUDE_DIR for the cross compiler.  Instead, set
  TARGET_SYSTEM_ROOT to point to WORLDTMP and always define
  STANDARD_INCLUDE_DIR.
- Change STANDARD_INCLUDE_DIR and the C++ include directories to just
  start with "/usr" always.  The compiler will prepend the sysroot when
  doing cross-builds.  GCC_INCLUDE_DIR (which contains headers that ship
  with the compiler such as intrinsincs rather than OS-supplied headers)
  remains hardcoded to look in TOOLS_PREFIX.

Reviewed by:	bdrewery (older version)
Sponsored by:	DARPA / AFRL
Differential Revision:	https://reviews.freebsd.org/D15127