perf tools: Allow cross compiling

For embedded platforms, we want to be able to build the perf
tools on a build machine to run on a different arch. This patch
allows $CROSS_COMPILE to set the cross compiler.

Additionally, if NO_LIBPERL is set, then don't use perl include
paths as they will be for the host arch.

Signed-off-by: Jamie Iles <jamie.iles@picochip.com>
Cc: Peter Zijlstra <peterz@infradead.org>
LKML-Reference: <1260523260-15694-2-git-send-email-jamie.iles@picochip.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>