x86: Move RETPOLINE*_CFLAGS to arch Makefile

Currently, RETPOLINE*_CFLAGS are defined in the top-level Makefile
but only x86 makes use of them. Move them there. If ever another
architecture finds the need, it can be reconsidered.

[ bp: Massage a bit. ]

Suggested-by: Nick Desaulniers <[e-mail]>
Signed-off-by: Peter Zijlstra (Intel) <[e-mail]>
Signed-off-by: Borislav Petkov <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Reviewed-by: Nick Desaulniers <[e-mail]>
Link: https://lkml.kernel.org/r/[e-mail]

Makefile

···
   CC_FLAGS_FTRACE := -pg
 endif
 
-ifdef CONFIG_CC_IS_GCC
-RETPOLINE_CFLAGS	:= $(call cc-option,-mindirect-branch=thunk-extern -mindirect-branch-register)
-RETPOLINE_VDSO_CFLAGS	:= $(call cc-option,-mindirect-branch=thunk-inline -mindirect-branch-register)
-endif
-ifdef CONFIG_CC_IS_CLANG
-RETPOLINE_CFLAGS	:= -mretpoline-external-thunk
-RETPOLINE_VDSO_CFLAGS	:= -mretpoline
-endif
-export RETPOLINE_CFLAGS
-export RETPOLINE_VDSO_CFLAGS
-
 include $(srctree)/arch/$(SRCARCH)/Makefile
 
 ifdef need-config

arch/x86/Makefile

···
         KBUILD_DEFCONFIG := $(ARCH)_defconfig
 endif
 
+ifdef CONFIG_CC_IS_GCC
+RETPOLINE_CFLAGS	:= $(call cc-option,-mindirect-branch=thunk-extern -mindirect-branch-register)
+RETPOLINE_VDSO_CFLAGS	:= $(call cc-option,-mindirect-branch=thunk-inline -mindirect-branch-register)
+endif
+ifdef CONFIG_CC_IS_CLANG
+RETPOLINE_CFLAGS	:= -mretpoline-external-thunk
+RETPOLINE_VDSO_CFLAGS	:= -mretpoline
+endif
+export RETPOLINE_CFLAGS
+export RETPOLINE_VDSO_CFLAGS
+
 # For gcc stack alignment is specified with -mpreferred-stack-boundary,
 # clang has the option -mstack-alignment for that purpose.
 ifneq ($(call cc-option, -mpreferred-stack-boundary=4),)