x86: fix race to build arch/x86/efi/relocs-dummy.o

x86: fix race to build arch/x86/efi/relocs-dummy.o

With $(TARGET).efi depending on efi/relocs-dummy.o, arch/x86/Makefile
will attempt to build that object. This may result in a dependency file
being generated that has relocs-dummy.o depending on efi/relocs-dummy.S.

Then, when arch/x86/efi/Makefile tries to build relocs-dummy.o, well
efi/relocs-dummy.S doesn't exist.

Have only one makefile responsible for building relocs-dummy.o.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Juergen Gross <jgross@suse.com>