modpost: Fix modpost license checking of vmlinux.o

Commit f02e8a65 ("module: Sort exported symbols") sorts symbols
placing each of them in its own elf section.  This sorting and merging
into the canonical sections are done by the linker.

Unfortunately modpost to generate Module.symvers file parses vmlinux.o
(which is not linked yet) 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.

[ This above is a slightly corrected version of the explanation of the
  problem, copied from commit 62a26356 ("modpost: Fix modpost's
  license checking V3").  That commit fixed the problem for module
  object files, but not for vmlinux.o.  This patch fixes modpost for
  vmlinux.o. ]

Signed-off-by: Frank Rowand <frank.rowand@am.sony.com>
Signed-off-by: Alessio Igor Bogani <abogani@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>