[NFC][llvm-objcopy] Fix overly restrictive od output check

The check against the output of `od` in the affected tests expect a
specific input offset format. They also expect a specific offset value,
not consistent with the EXAMPLE section for `od` in POSIX.1-2017
Chapter 4, while using the `-j` option. In particular, the example shows
that the input offset begins at 0 following the bytes skipped.

This patch adjusts the matching of the input offset to be more generic.
In order to avoid false matches, it restricts the number of bytes to be
formatted.

llvm-svn: 363829