Fix mimalloc init crash when Windows TLS is exhausted

[jinpzhanAMD]

Summary
This PR fixes a startup crash in the bundled mimalloc on Windows observed in TLS-slot-heavy processes. The crash occurred during mimalloc initialization when allocating “fast TLS user slots”.

Problem
mimalloc’s Windows fast-path TLS model relies on directly accessing the TEB TlsSlots[64] array. Current implementation assumed TlsAlloc() would return a TLS index that fits into the first 63 entries.
In environments where many DLLs/layers already consume TLS indices, TlsAlloc() returns an index >= 63, which cannot be addressed via the TEB's fixed 64-slot array, resulted in _mi_error_message(EFAULT, ...), causing a crash during mimalloc initialization.

Solution

• If the allocated TLS indices are >= 63, fall back to the Win32 TLS APIs (TlsGetValue/TlsSetValue) for correctness.
• If TlsAlloc() fails with TLS_OUT_OF_INDEXES, fall back to compiler TLS (__declspec(thread)) to store the default/cached theap pointers, avoiding reliance on OS TLS indices.
• Update the inline accessors in so reads and writes consistently follow the selected fallback mode.

Behavioral impact
Prevents mimalloc initialization from aborting in TLS-index-constrained processes.
Preserves the fast path when possible; uses progressively safer fallbacks only when required.
Expected minor performance impact only in the fallback modes.

[daanx]

Hi @jinpzhanAMD -- thanks again for putting in the work on this PR! The original code was indeed counting on not more than 64 TlsAlloc's and that is clearly too low a limit. I looked at the PR but we need to write this code quite carefully as it is all meant to improve the fast path over regular thread_local variables. I pushed a commit that now uses the Tls expansion slots when needed and this should work up to 1088 TlsAlloc's (which is the maximum anyway).

I think this will also address your other PR with the recursive initialization: that was an artifact of the initial limit as the initial slot was now containing an abritrary value instead of NULL. Let me know how it goes!