MINOR: build: define DEBUG_STRESS

MINOR: build: define DEBUG_STRESS

Define a new build mode DEBUG_STRESS. This will be used to stress some
code parts which cannot be reproduce easily with an alternative
suboptimal code.

First, a global <mode_stress> is set either to 1 or 0 depending on
DEBUG_STRESS compilation. A new global keyword "stress-level" is also
defined. It allows to specify a level from 0 to 9, to increase the
stress incurred on the code.

Helper macro STRESS_RUN* are defined for each stress level. This allows
to easily specify an instruction in default execution and a stress
counterpart if running on the corresponding stress level.