x86, arm64: Move ARCH_WANT_HUGE_PMD_SHARE config in arch/Kconfig

ARCH_WANT_HUGE_PMD_SHARE config was declared in both architectures:
move this declaration in arch/Kconfig and make those architectures
select it.

Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com> # for arm64
Reviewed-by: Hanjun Guo <guohanjun@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>