papabench: add compile-time flags that enable us to run and analyze papabench on patmos (and other platforms)

Change: Added compile-time flag -DNO_AVR_PLATFORM that accesses a volatile C object instead of
        hardcoded memory addresses; additionally, the attribute 'used' replace the attributes
        'signal' and 'interrupt', which are not available on all platforms
Rational: This allows to run PapaBench on other platforms than AVR; the current
          solution to this problem (access to a fixed memory address range, with configurable
          offset) requires custom linker scripts for each platform

Change: Added compile-time flag -DPAPABENCH_TEST, that executes each task
        one or more times with different test data.
Rational: Without this flag set, both executables run in an infinite loop, which is not
         suitable for testing/measurements in the simulator

Change: Added compile-time flag -DUNROLL_FP_LOOPS, that gets rid of loops
        that depend on floating-point values.
Rational: This is necessary for testing, as otherwise those loops might not
         (do not) terminate.

Change: Added functions for missing tasks in autopilot (the corresponding functions where
        described in the documentation, but missing in the source code)
Rational: We want to be able to analyze these functions

Change: Added missing return statement and #elif directive
Rational: silence warnings, no change in (defined) behavior