modpost: do not set ->preloaded for symbols from Module.symvers

Now that there is no overwrap between symbols from ELF files and
ones from Module.symvers.

So, the 'exported twice' warning should be reported irrespective
of where the symbol in question came from.

The exceptional case is external module; in some cases, we build
an external module to provide a different version/variant of the
corresponding in-kernel module, overriding the same set of exported
symbols.

You can see this use-case in upstream; tools/testing/nvdimm/libnvdimm.ko
replaces drivers/nvdimm/libnvdimm.ko in order to link it against mocked
version of core kernel symbols.

So, let's relax the 'exported twice' warning when building external
modules. The multiple export from external modules is warned only
when the previous one is from vmlinux or itself.

With this refactoring, the ugly preloading goes away.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>