strip: Add --reloc-debug-sections-only option.

strip: Add --reloc-debug-sections-only option.

This option does the same thing as --reloc-debug-sections without doing
any other strip operation. This is useful when you want to remove the
debug section relocations in a separate ET_REL debug file that was created
without --reloc-debug-sections, or for a file (like the linux debug vmlinux)
that you don't want to strip, but for which the debug section relocations
can be resolved already.

Signed-off-by: Mark Wielaard <mark@klomp.org>