Fix tcmalloc crashes on ARM64 Linux

[gxxk-dev]

tcmalloc assumes a 48-bit virtual address space, causing crashes on ARM64 systems with smaller VA space (e.g., 39-bit or 42-bit).

This PR restricts tcmalloc to x86_64 Linux only. ARM64 will use the default system allocator.

Tested on ARM64 Linux (Orange Pi RK3588):

• workerd builds and runs successfully
• 387 tests pass
• 29 timeouts are unrelated (due to slower ARM64 performance, tests pass when run individually)
• Hello World example works correctly

Fixes crashes with error: MmapAligned() failed - unable to allocate

[github-actions]

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[gxxk-dev]

I have read the CLA Document and I hereby sign the CLA

[codspeed-hq]

CodSpeed Performance Report

Merging #5571 will degrade performances by 8.33%

_{Comparing gxxk-dev:fix-tcmalloc-arm64 (071eace) with main (1413843)}

Summary

❌ 1 regression
✅ 56 untouched
⏩ 30 skipped¹

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
❌	simpleStringBody[Response]	21.4 µs	23.3 µs	-8.33%

Footnotes

1. 30 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩