19:20 jhb 

Various fixes to base/binutils and base/gcc6.

- Add --with-sysroot=/ to CONFIGURE_ARGS.  base/binutils needs this so
  that ld honors --sysroot in linker scripts.  base/gcc6 needs this so
  that the C++ include paths honor --sysroot.
- Remove a hack from GCC's configure to force C++98 mode when building
  GCC with GCC.  libc++ assumes C++11, so this hack was breaking the
  build of base/gcc6 natively when the system compiler was GCC.
- Add toolchain makefiles installed to /usr/share/toolchains.

  These are suitable to be used by CROSS_TOOLCHAIN when doing a native
  arch build.  These makefiles enable appropriate knobs so that the
  in-tree components that are replaced by each package are not built
  or installed.  The freebsd-gcc toolchain makefile includes the
  binutils toolchain makefile if it is present.

Reviewed by:	bapt
Differential Revision:	https://reviews.freebsd.org/D22984