Impact: minimize requirements on architectures

Currently, an architecture just enabling CONFIG_PERF_COUNTERS but not
providing any extra functions will fail to build with
perf_counter_print_debug being undefined, since we don't provide an
empty dummy definition like we do with the hw_perf_* functions.

This provides an empty dummy perf_counter_print_debug() to make it
easier for architectures to turn on CONFIG_PERF_COUNTERS.
Signed-off-by: Paul Mackerras <paulus@samba.org>