buildstats.py: enable collection of /proc/pressure data

buildstats.py: enable collection of /proc/pressure data

The Linux pressure monitoring system helps determine when system resources
are being overutilized by measuring how contended the CPU, IO and memory are.
This information can be found under /proc/pressure/ which contains 3 files -
cpu, memory and io. In each of the files, the format is as follows:
	some avg10=70.24 avg60=68.52 avg300=69.91 total=3559632828
	full avg10=57.59 avg60=58.06 avg300=60.38 total=3300487258

The "some" state of a given resource represents when one or more tasks are delayed
on that resource whereas the "full" state represents when all the tasks are
delayed. Currently, we only collect data from the "some" state but the
"full" data can simply be appended to the log files if neccessary.
The "avg10", "avg60" and "avg300" fields represent the average percentage
of time runnable tasks were delayed in the last 10, 60 or 300 seconds
respectively. The "total" field represents the total time, in microseconds,
that some runnable task was delayed on a resource.
More information can be found at:
	https://www.kernel.org/doc/html/latest/accounting/psi.html
and in the source code under kernel/sched/psi.c

This commit adds functionality to collect and log the "some" CPU, memory and IO
pressure. The "avg10", "avg60" and "avg300" fields are logged without change.
In place of the "total" field, the difference between the current "total" and
the previous sample's "total" is logged, allowing the measurement of pressure
in between each polling interval, as was done for /proc/stat data. The log files
are stored in:
	<build_name>/tmp/buildstats/<build_time>/reduced_proc_pressure/{cpu,io,memory}.log
mirroring the directory structure of /proc/pressure. If the /proc/pressure
directory does not exist or the resource files can't be read/opened, the
reduced_proc_pressure directory is not created.

Signed-off-by: Aryaman Gupta <aryaman.gupta@windriver.com>
Signed-off-by: Randy MacLeod <randy.macleod@windriver.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>