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.