x86/kbuild: Incorporate boot/startup/ via Kbuild makefile

Using core-y is not the correct way to get kbuild to descend into
arch/x86/boot/startup. For instance, building an individual object does
not work as expected when the pattern rule is local to the Makefile

  $ make arch/x86/boot/startup/map_kernel.pi.o
    GEN     Makefile
    CALL    /home/ardb/linux/scripts/checksyscalls.sh
    DESCEND objtool
    INSTALL libsubcmd_headers
  make[3]: *** No rule to make target 'arch/x86/boot/startup/map_kernel.pi.o'.  Stop.
  make[2]: *** [/home/ardb/linux/scripts/Makefile.build:461: arch/x86] Error 2
  make[1]: *** [/home/ardb/linux/Makefile:2011: .] Error 2
  make: *** [/home/ardb/linux/Makefile:248: __sub-make] Error 2

So use obj-y from arch.x86/Kbuild instead, which makes things work as
expected.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/20250828102202.1849035-42-ardb+git@google.com

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