soc/intel_adsp: Re-squash objcopy warnings

The fix for xcc build failures yesterday forgot to preserve the
squashing of stderr from the objcopy steps.  As mentioned in the
comments, there are deliberate empty sections in this link that
binutils warns about, but which are actually required, Cadence has
some vector sections in their standard exception table that don't get
populated and rimage has non-code sections that may be left empty for
non-audio-driver applications like tests.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>