perf tools: Fix build with ARCH=x86_64

With commit: 0a943cb10c (tools build: Add HOSTARCH Makefile variable)
when building for ARCH=x86_64, ARCH=x86_64 is passed to perf instead of
ARCH=x86, so the perf build process searchs header files from
tools/arch/x86_64/include, which doesn't exist.

The following build failure is seen:

  In file included from util/event.c:2:0:
    tools/include/uapi/linux/mman.h:4:27: fatal error: uapi/asm/mman.h: No such file or directory
    compilation terminated.

Fix this issue by using SRCARCH instead of ARCH in perf, just like the
main kernel Makefile and tools/objtool's.

Signed-off-by: Jiada Wang <jiada_wang@mentor.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Eugeniu Rosca <erosca@de.adit-jv.com>
Cc: Jan Stancek <jstancek@redhat.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ravi Bangoria <ravi.bangoria@linux.vnet.ibm.com>
Cc: Rui Teng <rui.teng@linux.vnet.ibm.com>
Cc: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Cc: Wang Nan <wangnan0@huawei.com>
Fixes: 0a943cb10c ("tools build: Add HOSTARCH Makefile variable")
Link: http://lkml.kernel.org/r/1491793357-14977-2-git-send-email-jiada_wang@mentor.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/arch/Build

@@ -1,2 +1,2 @@
 libperf-y += common.o
-libperf-y += $(ARCH)/
+libperf-y += $(SRCARCH)/