[llvm-objdump] Warn if no user specified sections (-j) are not found.

Match GNU objdump.

https://bugs.llvm.org/show_bug.cgi?id=41898

Reviewers: jhenderson, grimar, MaskRay, rupprecht

Reviewed by: jhenderson, grimar, MaskRay

Differential Revision: https://reviews.llvm.org/D63779

llvm-svn: 364955