riscv: Enable HAVE_ARCH_HUGE_VMALLOC for 64BIT

After we support HAVE_ARCH_HUGE_VMAP, we can now enable
HAVE_ARCH_HUGE_VMALLOC too. This feature has been used in kvmalloc and
alloc_large_system_hash for now. This feature can be disabled by kernel
parameters "nohugevmalloc".

Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Reviewed-by: Björn Töpel <bjorn@kernel.org>
Tested-by: Björn Töpel <bjorn@kernel.org>
Link: https://lore.kernel.org/r/20221012120038.1034354-3-liushixin2@huawei.com


[Palmer: minor formatting]
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>