Add a Makefile target for profiling targets

This simplifies the building of profiling-enabled htools binaries;
there are two targets, one used for the initial two-stage build, and
one that can be used for incremental builds.

The patch adds a new internal variable for the htools/test binary,
which should also simplify custom builds for the non-profiling
targets.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>