Bundle relocated interpreter's libdir into the dist toolchain package

[jetm]

Problem

sccache-dist packages a toolchain's shared libraries by parsing ldd
output, which resolves a binary's NEEDED libraries against the host's
dynamic loader. Yocto/OpenEmbedded "uninative" cross toolchains ship a
relocated glibc whose loader has a built-in search path pointing at its
own sysroot. For those binaries ldd reports host paths (e.g.
/usr/lib/libm.so.6), but inside the build sandbox the relocated loader
searches its own sysroot lib dir — where those libraries were never
packaged. The remote compile then dies with libm.so.6: cannot open shared object file and silently falls back to local compilation, so
distribution never actually runs.

Solution

When a packaged executable's PT_INTERP lives outside the standard host
loader directories (/lib, /lib64, /usr/lib, /usr/lib64), also
bundle the interpreter's own directory. That directory holds the
libc/libm the relocated loader resolves against, so they land at the
absolute path the loader searches inside the sandbox. Standard host
toolchains are unaffected: their interpreter is under a standard loader
directory, so the existing ldd-only path is preserved.

Key changes

• Add read_elf_interpreter() to read a binary's PT_INTERP via the
  already-vendored object crate.
• Add relocated_interpreter_libdir() to classify an interpreter as
  relocated and return the directory to bundle.
• In add_executable_and_deps(), bundle that directory's contents when a
  relocated interpreter is detected.
• Unit test covering standard vs. relocated interpreter classification.

Reviewer notes

• No behavior change for standard host toolchains — their interpreter is
  under /lib*, so nothing extra is bundled.
• Verified against a real OpenEmbedded aarch64 cross toolchain: a full
  Yocto image build distributed 634 compiles to the build server with 0
  dist errors and no libm.so.6 failures, and the packaged toolchain tar
  now contains the loader's libc/libm at the uninative sysroot path.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 82.97872% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.64%. Comparing base (b799af2) to head (082e7a0).

Files with missing lines	Patch %	Lines
src/dist/pkg.rs	82.97%	8 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2747   +/-   ##
=======================================
  Coverage   74.63%   74.64%           
=======================================
  Files          70       70           
  Lines       39898    39945   +47     
=======================================
+ Hits        29778    29817   +39     
- Misses      10120    10128    +8     

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[jetm]

Superseded by #2750, which combines this with the rest of the OpenEmbedded/Yocto distributed-compile fixes into a single series. Closing in favor of that PR.