pc-bios/s390-ccw/Makefile: Use -z noexecstack to silence linker warning

pc-bios/s390-ccw/Makefile: Use -z noexecstack to silence linker warning

Recent versions of ld complain when linking the s390-ccw bios:

 /usr/bin/ld: warning: start.o: missing .note.GNU-stack section implies
              executable stack
 /usr/bin/ld: NOTE: This behaviour is deprecated and will be removed in
              a future version of the linker

We can silence the warning by telling the linker to mark the stack
as not executable.

Message-Id: <20230622130822.396793-1-thuth@redhat.com>
Acked-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>