x86: Allow generating user-space headers without a compiler

When bootstrapping an architecture, it's usual to generate the kernel's
user-space headers (make headers_install) before building a compiler.  Move
the compiler check (for asm goto support) to the archprepare target so that
it is only done when building code for the target.

Fixes: e501ce957a ("x86: Force asm-goto")
Reported-by: Helmut Grohne <helmutg@debian.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20180829194317.GA4765@decadent.org.uk

arch/x86/Makefile

@@ -175,10 +175,6 @@ ifdef CONFIG_FUNCTION_GRAPH_TRACER
   endif
 endif
 
-ifndef CC_HAVE_ASM_GOTO
-  $(error Compiler lacks asm-goto support.)
-endif
-
 ifeq ($(ACCUMULATE_OUTGOING_ARGS), 1)
 	# This compiler flag is not supported by Clang:
 	KBUILD_CFLAGS += $(call cc-option,-maccumulate-outgoing-args,)
@@ -300,6 +296,13 @@ PHONY += vdso_install
 vdso_install:
 	$(Q)$(MAKE) $(build)=arch/x86/entry/vdso $@
 
+archprepare: checkbin
+checkbin:
+ifndef CC_HAVE_ASM_GOTO
+	@echo Compiler lacks asm-goto support.
+	@exit 1
+endif
+
 archclean:
 	$(Q)rm -rf $(objtree)/arch/i386
 	$(Q)rm -rf $(objtree)/arch/x86_64