For the same reason as commit 25896d07 ("x86/build: Fix compiler
support check for CONFIG_RETPOLINE"), you cannot put this $(error ...)
into the parse stage of the top Makefile.

Perhaps I'd propose a more sophisticated solution later, but this is
the best I can do for now.

Link: https://lkml.org/lkml/2017/12/25/211Reported-by: NPaul Gortmaker <paul.gortmaker@windriver.com>
Reported-by: NBernd Edlinger <bernd.edlinger@hotmail.de>
Reported-by: NQian Cai <cai@lca.pw>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: NQian Cai <cai@lca.pw>