Makefile: install modules.builtin even if CONFIG_MODULES=n

Many applications check for available kernel features via:

  - /proc/modules (loaded modules, present if CONFIG_MODULES=y)
  - $(MODLIB)/modules.builtin (builtin modules)

They fail to detect features if the kernel was built with CONFIG_MODULES=n
and modules.builtin isn't installed.

Therefore, add the target "_builtin_inst_" and make "install" and
"modules_install" depend on it.

Tests results:

  - make install: kernel image is copied as before, modules.builtin copied
  - make modules_install: (CONFIG_MODULES=n) nothing is copied, exit 1

Signed-off-by: Jonas Zeiger <jonas.zeiger@talpidae.net>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>