kbuild: disable header exports for UML in a straightforward way

kbuild: disable header exports for UML in a straightforward way

[ Upstream commit 1b620d539ccc18a1aca1613d9ff078115a7891a1 ]

Previously 'make ARCH=um headers' stopped because of missing
arch/um/include/uapi/asm/Kbuild.

The error is not shown since commit ed102bf2afed ("um: Fix W=1
missing-include-dirs warnings") added arch/um/include/uapi/asm/Kbuild.

Hard-code the unsupported architecture, so it works like before.

Fixes: ed102bf2afed ("um: Fix W=1 missing-include-dirs warnings")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Sasha Levin <sashal@kernel.org>