[llvm-nm] Allow --size-sort to print symbols with only Symbol size

Summary:
When llvm-nm is passed only the --size-sort option for an object file, there is no output generated.
The commit modifies the behavior to print the symbols sorted and their size which is also inline with
the output of the GNU nm tool.

Signed-off-by: Saurabh Badhwar <sbsaurabhbadhwar9@gmail.com>

Reviewers: enderby, rupprecht

Reviewed By: rupprecht

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D56063

llvm-svn: 351347