Fix race condition in memory_decommit/memory_reset on Apple ARM64

[Drustburn]

Summary

On Apple ARM64, memory_decommit and memory_reset used a munmap followed by mmap without MAP_FIXED (since Apple rejects MAP_FIXED | MAP_JIT). Between the two calls, another thread could claim the unmapped address range, causing mmap to return a different address and triggering a fatal verification error.

Under concurrent load (e.g. PPU LLVM compilation with many worker threads), this race manifests reliably as Verification failed (object: 0x0) crashes across all PPUW threads.

Fix

Use MAP_FIXED without MAP_JIT instead. This atomically replaces the mapping without any window for other threads to interfere. The MAP_JIT attribute is lost on the replaced pages, but the application's code signing entitlements (allow-unsigned-executable-memory, disable-executable-page-protection) permit executable mappings without it.

Before (crashes under load)

ensure(::munmap(pointer, size) != -1);
ensure(::mmap(pointer, size, PROT_NONE, MAP_ANON | MAP_PRIVATE | MAP_JIT, -1, 0) == pointer);

After (atomic, race-free)

ensure(::mmap(pointer, size, PROT_NONE, MAP_FIXED | MAP_ANON | MAP_PRIVATE | c_map_noreserve, -1, 0) != reinterpret_cast<void*>(uptr{umax}));

Test plan

• Tested on Apple M3 Max, macOS 26.3.1
• Verified all PPUW memory_decommit verification failures are eliminated
• PPU LLVM module loading completes without errors
• SPU interpreter builds successfully