objtool: Fix ARCH=x86_64 build error

Building objtool with ARCH=x86_64 fails with:

   $make ARCH=x86_64 -C tools/objtool
   ...
     CC       arch/x86/decode.o
   arch/x86/decode.c:10:22: fatal error: asm/insn.h: No such file or directory
    #include <asm/insn.h>
                         ^
   compilation terminated.
   mv: cannot stat ‘arch/x86/.decode.o.tmp’: No such file or directory
   make[2]: *** [arch/x86/decode.o] Error 1
   ...

The root cause is that the command-line variable 'ARCH' cannot be
overridden.  It can be replaced by 'SRCARCH', which is defined in
'tools/scripts/Makefile.arch'.
Signed-off-by: Shile Zhang <shile.zhang@linux.alibaba.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>
Link: https://lore.kernel.org/r/d5d11370ae116df6c653493acd300ec3d7f5e925.1579543924.git.jpoimboe@redhat.com

tools/objtool/Makefile

@@ -2,10 +2,6 @@
 include ../scripts/Makefile.include
 include ../scripts/Makefile.arch
 
-ifeq ($(ARCH),x86_64)
-ARCH := x86
-endif
-
 # always use the host compiler
 HOSTAR	?= ar
 HOSTCC	?= gcc
@@ -33,7 +29,7 @@ all: $(OBJTOOL)
 
 INCLUDES := -I$(srctree)/tools/include \
 	    -I$(srctree)/tools/arch/$(HOSTARCH)/include/uapi \
-	    -I$(srctree)/tools/arch/$(ARCH)/include
+	    -I$(srctree)/tools/arch/$(SRCARCH)/include
 WARNINGS := $(EXTRA_WARNINGS) -Wno-switch-default -Wno-switch-enum -Wno-packed
 CFLAGS   := -Werror $(WARNINGS) $(KBUILD_HOSTCFLAGS) -g $(INCLUDES) $(LIBELF_FLAGS)
 LDFLAGS  += $(LIBELF_LIBS) $(LIBSUBCMD) $(KBUILD_HOSTLDFLAGS)