llvm-objdump: Some style cleanups to follow LLVM coding style

Rename "ec" to "EC", and rename some iterators.

Then fix whitespace using clang-format-diff.

(As requested in http://llvm-reviews.chandlerc.com/D2559)

Differential Revision: http://llvm-reviews.chandlerc.com/D2594
Add a proper test for -Wsystem-headers

This will be extended as we fix issues in the current implementation.