autoPatchelfHook: fix detection under crossSystem
autoPatchelfHook: fix detection under crossSystem

In #84415, autoPatchelfHook was taught to use the correct path to the
readelf binary when a crossSystem is specified. Unfortunately, the
remainder of the functionality in the script depended on ldd, which only
reads ELF files of its own architecture. It has the further unfortunate
quality of not reporting any useful error, but rather that the file is
not a dynamic executable.

This change uses patchelf to directly analyze the DT_NEEDED tags in the
target files instead, which correctly works across architectures. It
also updates the use of objdump to be prefix-aware $OBJDUMP (which would
have been required in the PR mentioned above, but we never made it that
far into the script execution).

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