objtool: Use tools/scripts/Makefile.arch to get ARCH and HOSTARCH

objtool's Makefile was setting up ARCH but fixing up just the x86_64 ->
x86, using Makefile.arch will do the necessary fixups for all arches.

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-hbq0bbh03u2b722vozcyql31@git.kernel.orgSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>

tools/objtool/Makefile

 include ../scripts/Makefile.include
+include ../scripts/Makefile.arch
 
-ifndef ($(ARCH))
-ARCH ?= $(shell uname -m)
 ifeq ($(ARCH),x86_64)
 ARCH := x86
 endif
-endif
 
 # always use the host compiler
 CC = gcc