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