kbuild: move modules.builtin(.modinfo) rules to Makefile.vmlinux_o

Do not build modules.builtin(.modinfo) as a side-effect of vmlinux.

There are no good reason to rebuild them just because any of vmlinux's
prerequistes (vmlinux.lds, .vmlinux.export.c, etc.) has been updated.

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