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: f678068652 ("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>

arch/s390/boot/Makefile

@@ -58,7 +58,6 @@ define cmd_section_cmp
 	touch $@
 endef
 
-OBJCOPYFLAGS_bzImage := --pad-to $$(readelf -s $(obj)/compressed/vmlinux | awk '/\<_end\>/ {print or(strtonum("0x"$$2),4095)+1}')
 $(obj)/bzImage: $(obj)/compressed/vmlinux $(obj)/section_cmp.boot.data $(obj)/section_cmp.boot.preserved.data FORCE
 	$(call if_changed,objcopy)
 

arch/s390/boot/compressed/vmlinux.lds.S

@@ -77,6 +77,8 @@ SECTIONS
 		_compressed_start = .;
 		*(.vmlinux.bin.compressed)
 		_compressed_end = .;
+		FILL(0xff);
+		. = ALIGN(4096);
 	}
 	. = ALIGN(256);
 	.bss : {