tools lib lk: Respect CROSS_COMPILE

Make lk use CROSS_COMPILE, in order to be able to cross compile perf
again.
Signed-off-by: Rabin Vincent <rabin@rab.in>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Borislav Petkov <bp@suse.de>
Link: http://lkml.kernel.org/r/1368822464-4887-1-git-send-email-rabin@rab.inSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/lib/lk/Makefile

 include ../../scripts/Makefile.include
 
+CC = $(CROSS_COMPILE)gcc
+AR = $(CROSS_COMPILE)ar
+
 # guard against environment variables
 LIB_H=
 LIB_OBJS=