asm-generic: export: Stub EXPORT_SYMBOL with __DISABLE_EXPORTS

It is currently possible to stub EXPORT_SYMBOL() macros in C code using
__DISABLE_EXPORTS, which is necessary to run in constrained environments
such as the EFI stub or the decompressor. But this currently doesn't
apply to exports from assembly, which can lead to somewhat confusing
situations.

Consolidate the __DISABLE_EXPORTS infrastructure by checking it from
asm-generic/export.h as well.

Signed-off-by: Quentin Perret <qperret@google.com>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210203141931.615898-2-qperret@google.com