package/perf: use correct definition of ARCH on x86_64

LINUX_MAKE_FLAGS defines ARCH=$(KERNEL_ARCH), and KERNEL_ARCH is x86_64 on
a 64-bit x86 kernel build. The perf Makefiles expect that the ARCH will be
"x86" on both 32-bit x86 and 64-bit x86.

I didn't experience issues with the Linux 3.14.x version of perf, but this
issue cropped up once I started building 3.19.x.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
[Romain:
  - fix coding style]
Reviewed-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>