s390/purgatory: Make sure we fail the build if purgatory has missing symbols

Since we link purgatory with -r aka we enable "incremental linking"
no checks for unresolved symbols are done while linking the purgatory.

This commit adds an extra check for unresolved symbols by calling ld
without -r before running objcopy to generate purgatory.ro.

This will help us catch missing symbols in the purgatory sooner.

Note this commit also removes --no-undefined from LDFLAGS_purgatory
as that has no effect.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/lkml/20191212205304.191610-1-hdegoede@redhat.com


Tested-by: Philipp Rudo <prudo@linux.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>