perf tools: Makefile: Use gcc to determine ARCH

The original Makefile uses "uname -m" to determine ARCH.
This causes problem on x86 when compile perf tool on 32 bit
userspace with a 64 bit kernel.

 bench/../../../arch/x86/lib/memcpy_64.S: Assembler messages:
 bench/../../../arch/x86/lib/memcpy_64.S:28: Error: bad register name `%rdi'

This is because "uname -m" returns x86_64 and memcpy_64.S is
included in 32 bit build.

Reported-by: Riccardo Magliocchetti <riccardo.magliocchetti@gmail.com>
Signed-off-by: Lin Ming <ming.m.lin@intel.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@infradead.org>
Link: http://lkml.kernel.org/r/1304743274.3132.17.camel@localhost
Signed-off-by: Ingo Molnar <mingo@elte.hu>