mi_reserve_os_memory fails to use 2MB hugepages when reserving >1GB memory

Nobody has claimed this yet.

Hi,

I am currently using a Linux system where the only hugepage size available is 2MB. With mimalloc 2.0.9, when I tried to use `mi_reserve_os_memory` to reserve 4GB memory, I got the following warning message:
`mimalloc: warning: unable to allocate aligned OS memory directly, fall back to over-allocation (4294967296 bytes, address: 0x7ffef7200000, alignment: 33554432, commit: 1)`

It appears that `mi_os_get_aligned_hint` always return NULL for `size` > 1GB (I understand this behavior is related to #372). Thus, without an alignment hint, the result of `mi_os_mem_alloc` is unlikely aligned to `MI_SEGMENT_ALIGN` (32MB), which results in the fall back to over-allocation.

Is there a workaround to achieve my goal? Or should I configure the system to enable 1GB hugepages? Thank you!

Start by tracing mi_reserve_os_memory through mi_os_get_aligned_hint and mi_os_mem_alloc, focusing on the >1GB case and the MI_SEGMENT_ALIGN requirement. Review the related behavior in #372 and reproduce the Linux configuration with only 2MB hugepages; done means determining whether 4GB reservation can avoid over-allocation or documenting the required workaround.

Written by the indexing model from the issue text.