tools build: Add support for host programs format

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 <[email protected]>
Requested-by: Andi Kleen <[email protected]>
Requested-and-Tested-by: Arnaldo Carvalho de Melo <[email protected]>
Cc: Madhavan Srinivasan <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Sukadev Bhattiprolu <[email protected]>
Cc: [email protected]
Link: http://lkml.kernel.org/r/20160927141846.GA6589@krava
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>