LATX, fix: Resolve Steam i386 libraries beside selected

[baibaidashixiong]

Summary / 变更说明

• Title: linux-user, fix: Resolve Steam i386 libraries beside selected loader

Steam Runtime i386 steam and gldriverquery can request an absolute
lib*.so* that is absent from the configured runtime prefix even though the
selected PT_INTERP loader's directory contains the matching library. This
prevented steam from creating its GLX visual and blocked the client UI.

After the original read-only unprefixed open returns ENOENT, this PR retries
the same library basename beside the selected loader only for steam or
gldriverquery. It reuses the existing trusted loader-page check, retains the
original errno on an unsuccessful retry, and keeps the fallback i386-only. It
adds no launcher, LAT_SET_ENV, PRESSURE_VESSEL_PREFIX, or other environment
injection.

No dedicated automated test is added: a faithful fixture requires an i386
guest, a selected PT_INTERP loader, and a deliberately incomplete runtime
prefix. Steam Runtime gldriverquery directly exercises the affected path.
Independent review passed after the fallback was limited to the two observed
Steam executable basenames.

Validation / 验证

• ninja -C build32 --quiet latx-i386: passed.
• ninja -C build64 --quiet latx-x86_64: passed.
• git diff --check: passed.
• Commit-level scripts/checkpatch.pl: 0 errors, 0 warnings.
• Steam Runtime regression:
  ubuntu12_32/steam-runtime/run.sh ubuntu12_32/gldriverquery passed the
  previous “Couldn't find matching GLX visual” failure and reached the
  separate radeonsi / X_GLXCreateContext failure.

Remaining Scope / 风险

• Only the steam and gldriverquery basenames executing through a trusted
  i386 dynamic loader may invoke the fallback, and only for a read-only
  absolute lib*.so* open that missed the normal unprefixed path.
• A library absent beside the selected loader returns the original ENOENT.
• The fallback path is compiled out for x86_64; the shared loader identity
  helper remains available to its pre-existing x86_64 tunnel notification use.
• No negative test currently covers other executable basenames, an untrusted
  loader page, retry errno restoration, or O_PATH; the independent review
  accepted this as a narrow, Steam-only limitation rather than widening the
  change or adding an environment-specific fixture.
• This is a hack method specifically for Steam, not applicable to general scenarios.

Checklist / 检查项

• I have read CONTRIBUTING.md. / 我已阅读 CONTRIBUTING.md。
• Every commit contains a DCO sign-off (git commit -s). /
  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。