perf/test: Speed up running brstack test on an Arm model

The test runs quite slowly in the model, so replace "xargs -n1" with
"tr ' ' '\n'" which does the same thing but in single digit minutes
instead of double digit minutes.

Also reduce the number of loops in the test application. Unfortunately
this causes intermittent failures on x86, presumably because the
sampling interval is too big to pickup any loops, so keep it the same
there.

Signed-off-by: James Clark <james.clark@arm.com>
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>