macros, build: override find-debuginfo location

[bcressey]

Issue number:
N/A

Description of changes:
This fixes a regression where find-debuginfo now prefers objcopy in /usr/bin rather than the target-specific symlink placed ahead of it in $PATH. That happens because find-debuginfo now adds its own parent directory to $PATH in front of everything else.

Work around this by adding a find-debuginfo symlink to the debug helper directory, and override the macro so that find-debuginfo is called from that directory.

Also add a readelf symlink, since find-debuginfo calls it too.

Testing done:
Before, the output logs would show errors like this when building on x86_64 for aarch64:

  #14 133.7 find-debuginfo: starting
  #14 133.8 Extracting debug info from 9 files
  #14 134.9 objcopy: Unable to recognise the format of the input file `/home/builder/rpmbuild/BUILDROOT/bottlerocket-settings-plugins-0.0-0.1727497932.29b947  0bc.br1.x86_64/aarch64-bottlerocket-linux-gnu/sys-root/usr/lib/settings-plugins/aws-dev/libsettings.so'

After, the errors go away since the target-specific objcopy is being used.

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[bcressey]

This is what feels like the millionth time I've had to fix debuginfo extraction in our cross-compilation setup.

That said, we will now run find-debuginfo from a specially-prepared, target-specific directory with controlled symlinks that point to the correct binutils programs to use. So this change should be just about as self-contained and focused on the precise use case - debuginfo extraction - as is possible, without the negative side effects of cluttering $PATH during the build.

See bottlerocket-os/bottlerocket-core-kit#174 for a related fix that this helped to catch, after the cleanup in bottlerocket-os/twoliter#383.