s390/boot: fix compiler error due to missing awk strtonum

The strtonum awk function is a GNU extension and is not available with
all versions of awk. The link of bzImage fails with this error message:

>> awk: line 2: function or never defined
>> awk: line 2: function strtonum never defined
   objcopy: --pad-to: bad number: arch/s390/boot/compressed/vmlinux

Drop the awk script and the --pad-to objcopy parameter it generated and
use a FILL pattern with an appropriate alignment in the linker script
for the arch/s390/boot/compressed/vmlinux file.

Fixes: f6780686 ("s390/boot: pad bzImage to 4K")
Reported-by: kbuild test robot <lkp@intel.com>
Suggested-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>