x86/build: Fix compiler support check for CONFIG_RETPOLINE

It is troublesome to add a diagnostic like this to the Makefile
parse stage because the top-level Makefile could be parsed with
a stale include/config/auto.conf.

Once you are hit by the error about non-retpoline compiler, the
compilation still breaks even after disabling CONFIG_RETPOLINE.

The easiest fix is to move this check to the "archprepare" like
this commit did:

829fe4aa9ac1 ("x86: Allow generating user-space headers without a compiler")

Reported-by: Meelis Roos <[e-mail]>
Tested-by: Meelis Roos <[e-mail]>
Signed-off-by: Masahiro Yamada <[e-mail]>
Acked-by: Zhenzhong Duan <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: Zhenzhong Duan <[e-mail]>
Fixes: 4cd24de3a098 ("x86/retpoline: Make CONFIG_RETPOLINE depend on compiler support")
Link: http://lkml.kernel.org/r/[e-mail]
Link: https://lkml.org/lkml/2018/12/4/206
Signed-off-by: Ingo Molnar <[e-mail]>

arch/x86/Makefile

···
 
 # Avoid indirect branches in kernel to deal with Spectre
 ifdef CONFIG_RETPOLINE
-ifeq ($(RETPOLINE_CFLAGS),)
-  $(error You are building kernel with non-retpoline compiler, please update your compiler.)
-endif
   KBUILD_CFLAGS += $(RETPOLINE_CFLAGS)
 endif
 
···
 ifndef CC_HAVE_ASM_GOTO
 	@echo Compiler lacks asm-goto support.
 	@exit 1
+endif
+ifdef CONFIG_RETPOLINE
+ifeq ($(RETPOLINE_CFLAGS),)
+	@echo "You are building kernel with non-retpoline compiler." >&2
+	@echo "Please update your compiler." >&2
+	@false
+endif
 endif
 
 archclean: