modpost: Fix modpost's license checking V3

The commit f02e8a65 sorts symbols placing each of them in its own elf section.
The sorting and merging into the canonical sections are done by the linker.
Unfortunately modpost to generate Module.symvers file parses vmlinux
(already linked) and all modules object files (which aren't linked yet).
These aren't sanitized by the linker yet. That breaks modpost that can't
detect license properly for modules. This patch makes modpost aware of
the new exported symbols structure.

Thanks to Arnaud Lacombe <lacombar@gmail.com> and Anders Kaseorg
<andersk@ksplice.com> for providing useful suggestions about code.

This work was supported by a hardware donation from the CE Linux Forum.

Reported-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Alessio Igor Bogani <abogani@kernel.org>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>