kbuild: modpost on vmlinux regardless of CONFIG_MODULES

Based on patch from: Magnus Damm <magnus@valinux.co.jp>
This has the advantage that all section mismatch checks are run regardless
of modules being enabled or not.

When running modpost on vmlinux output:
MODPOST vmlinux

When running modpost on modules output count of modules like this:
MODPOST 5 modules

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>