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 <[e-mail]>
Signed-off-by: Josh Poimboeuf <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>
Reviewed-by: Kamalesh Babulal <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

tools/objtool/Makefile

···
 include ../scripts/Makefile.include
 include ../scripts/Makefile.arch
 
-ifeq ($(ARCH),x86_64)
-ARCH := x86
-endif
-
 # always use the host compiler
 HOSTAR	?= ar
 HOSTCC	?= gcc
···
 
 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)