linux/perf: honour the number of parallel jobs

linux/perf: honour the number of parallel jobs

perf does not honour the -j flags we pass to make; it yet again tries to
reinvent the wheel and by default uses the number of CPUs as the number
of parallel jobs.

Fortunately, in their infinite wisdom, the insane developpers of the
perf buildsystem were kind enough to provide us with a variable we can
set to specify the number of parallel jobs.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>