version --build-options: also report host CPU

version --build-options: also report host CPU

It can be helpful for bug reports to include information about the
environment in which the bug occurs. "git version --build-options" can
help to supplement this information. In addition to the size of 'long'
already reported by --build-options, also report the host's CPU type.
Example output:

   $ git version --build-options
   git version 2.9.3.windows.2.826.g06c0f2f
   cpu: x86_64
   sizeof-long: 4

New Makefile variable HOST_CPU supports cross-compiling.

Suggested-by: Adric Norris <landstander668@gmail.com>
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>