libperf: Fix arch include paths

Guenter Roeck reported problem with compilation when the ARCH is
specified:

  $ make ARCH=x86_64
  In file included from tools/include/asm/atomic.h:6:0,
                   from include/linux/atomic.h:5,
                   from tools/include/linux/refcount.h:41,
                   from cpumap.c:4: tools/include/asm/../../arch/x86/include/asm/atomic.h:11:10:
  fatal error: asm/cmpxchg.h: No such file or directory

The problem is that we don't use SRCARCH (the sanitized ARCH version)
and we don't get the proper include path.

Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexey Budankov <alexey.budankov@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Fixes: 31435049 ("libperf: Make libperf.a part of the perf build")
Link: http://lkml.kernel.org/r/20190820124624.GG24105@krava


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>