x86: Get rid of Makefile.postlink

Instead of generating the vmlinux.relocs file (needed by the
decompressor build to construct the KASLR relocation tables) as a
vmlinux postlink step, which is dubious because it depends on data that
is stripped from vmlinux before the build completes, generate it from
vmlinux.unstripped, which has been introduced specifically for this
purpose.

This ensures that each artifact is rebuilt as needed, rather than as a
side effect of another build rule.

This effectively reverts commit

  9d9173e9 ("x86/build: Avoid relocation information in final vmlinux")

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>