Update gdb performance testsuite to be compatible with Python 3.8

Update gdb performance testsuite to be compatible with Python 3.8

Running "make check-perf" on a system with Python 3.8 (e.g., Ubuntu
20.04) runs into this Python problem:

  Traceback (most recent call last):
    File "<string>", line 1, in <module>
    File "/home/pedro/rocm/gdb/src/gdb/testsuite/gdb.perf/lib/perftest/perftest.py", line 65, in run
      self.execute_test()
    File "<string>", line 35, in execute_test
    File "/home/pedro/rocm/gdb/src/gdb/testsuite/gdb.perf/lib/perftest/measure.py", line 45, in measure
      m.start(id)
    File "/home/pedro/rocm/gdb/src/gdb/testsuite/gdb.perf/lib/perftest/measure.py", line 102, in start
      self.start_time = time.clock()
  AttributeError: module 'time' has no attribute 'clock'
  Error while executing Python code.
  (gdb) FAIL: gdb.perf/single-step.exp: python SingleStep(1000).run()

... many times over.

The problem is that the testsuite is using time.clock(), deprecated in
Python 3.3 and finaly removed in Python 3.8.  The guidelines say to
use time.perf_counter() or time.process_time() instead depending on
requirements.  Looking at the current description of those functions,
at:

   https://docs.python.org/3.10/library/time.html

we have:

   time.perf_counter() -> float

       Return the value (in fractional seconds) of a performance
       counter, i.e. a clock with the highest available resolution to
       measure a short duration. It does include time elapsed during
       sleep and is system-wide. (...)

   time.process_time() -> float

       Return the value (in fractional seconds) of the sum of the
       system and user CPU time of the current process. It does not
       include time elapsed during sleep. It is process-wide by
       definition. (...)

I'm thinking that it's just best to record both instead of picking
one.  So this patch replaces the MeasurementCpuTime measurement class
with two new classes -- MeasurementPerfCounter and
MeasurementProcessTime.  Correspondingly, this changes the reports in
testsuite/perftest.log -- we have two new "perf_counter" and
"process_time" measurements and the "cpu_time" measurement is gone.  I
don't suppose breaking backward compatibility here is a big problem.
I suspect no one is really tracking long term performance using the
perf testsuite today.  And if they are, it shouldn't be hard to adjust.

For backward compatility, with Python < 3.3, both perf_counter and
process_time use the old time.clock.

gdb/testsuite/ChangeLog:
yyyy-mm-dd  Qingchuan Shi  <qingchuan.shi@amd.com>
	    Pedro Alves  <pedro@palves.net>

	* gdb.perf/lib/perftest/perftest.py: Import sys.
	(time.perf_counter, time.process_time): Map to time.clock on
	Python < 3.3.
	(MeasurementCpuTime): Delete, replaced by...
	(MeasurementPerfCounter, MeasurementProcessTime): .. these two new
	classes.
	* gdb.perf/lib/perftest/perftest.py: Import MeasurementPerfCounter
	and MeasurementProcessTime instead of MeasurementCpuTime.
	(TestCaseWithBasicMeasurements): Use MeasurementPerfCounter and
	MeasurementProcessTime instead of MeasurementCpuTime.

Co-authored-by: Qingchuan Shi <qingchuan.shi@amd.com>
Change-Id: Ia850c05d5ce57d2dada70ba5b0061f566444aa2b