tools build: Add support for host programs format

In some cases, like for fixdep and shortly for jevents, we need to build a tool
to run on the host that will be used in building a tool, such as perf, that is
being cross compiled, so do like the kernel and provide HOSTCC, HOSTLD and HOSTAR
to do that.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Requested-by: Andi Kleen <andi@firstfloor.org>
Requested-and-Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Cc: linuxppc-dev@lists.ozlabs.org
Link: http://lkml.kernel.org/r/20160927141846.GA6589@krava
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>