The objtool build fails in a cross-compiled environment on a non-x86
host with "ARCH=x86_64":

  tools/objtool/objtool-in.o: In function `decode_instructions':
  tools/objtool/builtin-check.c:276: undefined reference to `arch_decode_instruction'

We could override the ARCH environment variable and change it back to
x86, similar to what the objtool Makefile was doing before; but it's
tricky to override environment variables consistently.

Instead, take a similar approach used by the Linux top-level Makefile
and introduce a SRCARCH Makefile variable which evaluates to "x86" when
ARCH is either "x86_64" or "x86".

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20160722191920.ej62fnspnqurbaa7@treble
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

For tools that needs to be always compiled with the host headers.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-907q32k2nep6q670dkxypmu6@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

After this patch other directories can use this architecture detector
without directly including it from perf's directory. Libbpf would
utilize it to get proper $(ARCH) so it can receive correct uapi include
directory.

Tested-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Cc: Zefan Li <lizefan@huawei.com>
Cc: pi3orama@163.com
Signed-off-by: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1452520124-2073-8-git-send-email-wangnan0@huawei.com
[ Add missing srctree definition in tests/make ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@kernel.org>