Add argv[0] fallback for current_exe() on Linux when /proc is unavailable

[Ecordonnier]

When /proc/self/exe is not accessible (e.g., in containers with masked /proc, chroot environments, or systemd services with ProtectProc=invisible), current_exe() will now fall back to parsing argv[0] and searching PATH.

This brings the Linux implementation in line with existing Rust stdlib patterns on Fuchsia, Solaris/illumos, and AIX, which also use argv[0] when direct kernel APIs are unavailable.

Fallback strategy:

1. Try /proc/self/exe (fast path, existing behavior)
2. On NotFound error, retrieve argv[0] from env::args()
3. If argv[0] is absolute, use it directly
4. If argv[0] contains '/', resolve it as relative path against getcwd()
5. Otherwise, search PATH for the executable (checking execute permissions)
6. Return NotFound error if executable not found in PATH

This handles all common invocation patterns:

• Absolute path: /usr/bin/program
• Relative path: ./program or ../bin/program
• PATH lookup: program (searches directories in PATH)

This maintains backward compatibility: behavior is unchanged when /proc is accessible.

Fixes #46090

[rustbot]

r? @Mark-Simulacrum

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• Owners of files modified in this PR: @ChrisDenton, libs
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[Mark-Simulacrum]

Nominating for libs-api to discuss whether we want this or not. #152269 has some good discussion that is worth reading for that discussion, in particular #152269 (comment).

[Ecordonnier]

Nominating for libs-api to discuss whether we want this or not. #152269 has some good discussion that is worth reading for that discussion, in particular #152269 (comment).

Please also consider #46090 in the discussion. The comments in #152269 support the opinion that the corner-case of /proc not being present doesn't exist, while #46090 shows a real-world example of this corner-case.

[rust-bors]

☔ The latest upstream changes (presumably #154255) made this pull request unmergeable. Please resolve the merge conflicts.