How to cross build initial toolchain

Example with sparc64

1/ install a cross toolchain
pkg install sparc64-gcc6

2/ cross build world
make CROSS_TOOLCHAIN=sparc64-gcc6 TARGET=sparc64 TARGET_ARCH=sparc64 buildworld

3/ install the world in a sysroot
make CROSS_TOOLCHAIN=sparc64-gcc6 TARGET=sparc64 TARGET_ARCH=sparc64 installworld DESTDIR=/sysroots/sparc64

4/ build the required port
$ cd base/binutils
make CROSS_TOOLCHAIN=sparc64-gcc6 CROSS_SYSROOT=/sysroots/sparc64 package

$ cd base/gcc6
make CROSS_TOOLCHAIN=sparc64-gcc6 CROSS_SYSROOT=/sysroots/sparc64 package

The packages are in base/gcc6/work/pkg/*.txz and base/binutils/work/pkg/*.txz

To add a plist for a new binutils target
$ cd base/binutils
make CROSS_TOOLCHAIN=sparc64-gcc6 CROSS_SYSROOT=/sysroots/sparc64 makeplist > pkg-plist

edit the pkg-plist and remove everything which base does not provide via the
elftoolchain