x86/build: Use obj-y to descend into arch/x86/virt/

Commit c33621b4 ("x86/virt/tdx: Wire up basic SEAMCALL functions")
introduced a new instance of core-y instead of the standardized obj-y
syntax.

X86 Makefiles descend into subdirectories of arch/x86/virt inconsistently;
into arch/x86/virt/ via core-y defined in arch/x86/Makefile, but into
arch/x86/virt/svm/ via obj-y defined in arch/x86/Kbuild.

This is problematic when you build a single object in parallel because
multiple threads attempt to build the same file.

  $ make -j$(nproc) arch/x86/virt/vmx/tdx/seamcall.o
    [ snip ]
    AS      arch/x86/virt/vmx/tdx/seamcall.o
    AS      arch/x86/virt/vmx/tdx/seamcall.o
  fixdep: error opening file: arch/x86/virt/vmx/tdx/.seamcall.o.d: No such file or directory
  make[4]: *** [scripts/Makefile.build:362: arch/x86/virt/vmx/tdx/seamcall.o] Error 2

Use the obj-y syntax, as it works correctly.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20240330060554.18524-1-masahiroy@kernel.org

arch/x86/Kbuild

@@ -28,7 +28,7 @@ obj-y += net/
 
 obj-$(CONFIG_KEXEC_FILE) += purgatory/
 
-obj-y += virt/svm/
+obj-y += virt/
 
 # for cleaning
 subdir- += boot tools

arch/x86/Makefile

@@ -251,8 +251,6 @@ archheaders:
 
 libs-y  += arch/x86/lib/
 
-core-y += arch/x86/virt/
-
 # drivers-y are linked after core-y
 drivers-$(CONFIG_MATH_EMULATION) += arch/x86/math-emu/
 drivers-$(CONFIG_PCI)            += arch/x86/pci/

arch/x86/virt/Makefile

 # SPDX-License-Identifier: GPL-2.0-only
-obj-y	+= vmx/
+obj-y	+= svm/ vmx/