[llvm-readobj] Add a flag to dump just the section-to-segment mapping.

Summary:
The following patch introduces a new function `printSectionMapping` which is responsible for dumping just the section-to-segment mapping.
This patch also introduces a n option `-section-mapping` that outputs that mapping without the program headers.

Previously, this functionality was controlled by `printProgramHeaders`, and the output from `-program-headers` has not been changed.  I am happy to change the option name, I copied  the name that was displayed when outputting the mapping table.

Reviewers: khemant, jhenderson, grimar, rupprecht

Reviewed By: jhenderson, grimar, rupprecht

Subscribers: rupprecht, jhenderson, llvm-commits

Differential Revision: https://reviews.llvm.org/D57365

llvm-svn: 352896