LATX, fix: Release file streams on config and ELF exit paths

[baibaidashixiong]

Summary / 变更说明

Fix file-descriptor leaks in LATX configuration parsing and ELF loading.
The valid parsing and library-loading paths keep their existing behavior and
ordering; file resources are only released after they are no longer needed or
when an error path returns early.

修复 LATX 配置解析和 ELF 加载路径中的文件描述符泄漏。正常配置解析和库加载
行为、查找顺序保持不变，只在资源不再需要或错误路径提前返回时释放文件资源。

• 5ef3b671 load_conf_file(): close the configuration FILE after the
  parsing loop so normal EOF, early section break, and read-error exits release
  the descriptor. Each process opens at most the system and user config files.
• 299ea156 loadElfFromFile(): close the ELF stream after
  LoadAndCheckElfHeader(), return NULL when header parsing fails, and clear
  the stored FILE/fd metadata before returning the header.
• c4bc33db find_elf_rpath(): close the stream on stat() and header-parse
  failures, and close the raw descriptor when fdopen() fails.

Validation / 验证

• Rebased onto lat-opensource/lat master 7cbdeb8f.
• ./latxbuild/build-release.sh: passed.
• meson test -C build64-tests-nokzt --suite lat-pr-fast --print-errorlogs:
  43/43 passed, 0 failed.
• LoongArch runtime smoke tests with LATX_KZT=1 LATX_AOT=0:
  • /usr/gnemul/lat-x86_64/bin/true: exit status 0
  • dlopen("libm.so.6") guest: printed 1.000
  • guest with $ORIGIN/lib RPATH: printed 42
• git diff --check: passed.

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。