MI_HINT area is outside the VA range on some systems

https://github.com/microsoft/mimalloc/blob/f2712f4a8f038a7fb4df2790f4c3b7e3ed9e219b/src/os.c#L315-L317

MI_HINT_BASE is defined to be 2 TiB, but on some systems this is outside the VA range.

In particular, the AArch64 default config in Linux selects a 39-bit VA size (512GB), though many distributions do use 48-bit VAs. There is also an option for 36-bit addresses, though that is guarded behind EXPERT and unlikely to be widely used.

RISC-V also appears to limit VAs to 39 bits in some situations.

Because of this, any program linking mimalloc on an affected system will print this warning:

$ LD_PRELOAD=libmimalloc.so.2.0 ls
mimalloc: warning: unable to allocate aligned OS memory directly, fall back to over-allocation (67108864 bytes, address: 0x7f8fded000, alignment: 67108864, commit: 1)

Start in src/os.c at lines 315-317 and trace how MI_HINT_BASE is used when requesting aligned OS memory. Compare that assumption with the AArch64 and RISC-V virtual-address limits described in the issue, including the 39-bit Linux configuration. Done means the affected systems no longer produce the reported fallback warning while preserving the intended aligned allocation behavior.