linux-user, fix: Limit private mmap shadows to SQLite

[ganjue66da]

Wine maps SQLite database files privately. The host-page fragment fallback copies data with pread into anonymous memory, so later writes through the file descriptor are not visible. WXWork can then reopen a document approval as a white view.

The initial opt-in shadow path covered every writable file descriptor. On UOS this also shadowed writable PE cache files. WXWork could then fault in over_page_write during startup.

Require MAP_PRIVATE and a writable descriptor whose first 16 bytes match the SQLite database header. Other writable files keep the normal fragment fallback, while SQLite fragments remain file-backed when LATX_PRIVATE_MMAP_SHADOW=1.

Tests:

• UOS WXWork 5.0.0.6008: launch and reopen one document approval
• Linux and Wine SQLite regression: 10,000 mmap query/update iterations

Summary / 变更说明

Validation / 验证

Checklist / 检查项

• I have read CONTRIBUTING.md. / 我已阅读 CONTRIBUTING.md。
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  每个提交都包含 DCO 签署（git commit -s）。
• I have included relevant build or test results, or explained why they
  are not applicable. /
  我已提供相关构建或测试结果，或说明了不适用的原因。