s390/build: Avoid relocation information in final vmlinux

Since commit 778666df ("s390: compile relocatable kernel without
-fPIE") the kernel vmlinux ELF file is linked with --emit-relocs to
preserve all relocations, so that all absolute relocations can be
extracted using the 'relocs' tool to adjust them during boot.

Port and adapt Petr Pavlu's x86 commit 9d9173e9 ("x86/build: Avoid
relocation information in final vmlinux") to s390 to strip all
relocations from the final vmlinux ELF file to optimize its size.
Following is his original commit message with minor adaptions for s390:

The Linux build process on s390 roughly consists of compiling all input
files, statically linking them into a vmlinux ELF file, and then taking
and turning this file into an actual bzImage bootable file.

vmlinux has in this process two main purposes:
1) It is an intermediate build target on the way to produce the final
   bootable image.
2) It is a file that is expected to be used by debuggers and standard
   ELF tooling to work with the built kernel.

For the second purpose, a vmlinux file is typically collected by various
package build recipes, such as distribution spec files, including the
kernel's own tar-pkg target.

When building the kernel vmlinux contains also relocation information
produced by using the --emit-relocs linker option. This is utilized by
subsequent build steps to create relocs.S and produce a relocatable
image. However, the information is not needed by debuggers and other
standard ELF tooling.

The issue is then that the collected vmlinux file and hence distribution
packages end up unnecessarily large because of this extra data. The
following is a size comparison of vmlinux v6.10 with and without the
relocation information:

  | Configuration      | With relocs | Stripped relocs |
  | defconfig          |      696 MB |          320 MB |
  | -CONFIG_DEBUG_INFO |       48 MB |           32 MB |

Optimize a resulting vmlinux by adding a postlink step that splits the
relocation information into relocs.S and then strips it from the vmlinux
binary.
Reviewed-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Jens Remus <jremus@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/Makefile.postlink

+# SPDX-License-Identifier: GPL-2.0
+# ===========================================================================
+# Post-link s390 pass
+# ===========================================================================
+#
+# 1. Separate relocations from vmlinux into relocs.S.
+# 2. Strip relocations from vmlinux.
+
+PHONY := __archpost
+__archpost:
+
+-include include/config/auto.conf
+include $(srctree)/scripts/Kbuild.include
+
+CMD_RELOCS=arch/s390/tools/relocs
+OUT_RELOCS = arch/s390/boot
+quiet_cmd_relocs = RELOCS  $(OUT_RELOCS)/relocs.S
+      cmd_relocs = \
+	mkdir -p $(OUT_RELOCS); \
+	$(CMD_RELOCS) $@ > $(OUT_RELOCS)/relocs.S
+
+quiet_cmd_strip_relocs = RSTRIP  $@
+      cmd_strip_relocs = \
+	$(OBJCOPY) --remove-section='.rel.*' --remove-section='.rel__*' \
+		   --remove-section='.rela.*' --remove-section='.rela__*' $@
+
+vmlinux: FORCE
+	$(call cmd,relocs)
+	$(call cmd,strip_relocs)
+
+clean:
+	@rm -f $(OUT_RELOCS)/relocs.S
+
+PHONY += FORCE clean
+
+FORCE:
+
+.PHONY: $(PHONY)

arch/s390/boot/Makefile

@@ -109,11 +109,9 @@ OBJCOPYFLAGS_vmlinux.bin := -O binary --remove-section=.comment --remove-section
 $(obj)/vmlinux.bin: vmlinux FORCE
 	$(call if_changed,objcopy)
 
-CMD_RELOCS=arch/s390/tools/relocs
-quiet_cmd_relocs = RELOCS  $@
-	cmd_relocs = $(CMD_RELOCS) $< > $@
-$(obj)/relocs.S: vmlinux FORCE
-	$(call if_changed,relocs)
+# relocs.S is created by the vmlinux postlink step.
+$(obj)/relocs.S: vmlinux
+	@true
 
 suffix-$(CONFIG_KERNEL_GZIP)  := .gz
 suffix-$(CONFIG_KERNEL_BZIP2) := .bz2