[ELF] - ICF: Add test case to check many sections case.

Currently, ICF does not enable threading if we have less than 1024
sections in each equivalence class.

And the following code is uncovered by our test cases:
https://github.com/llvm-mirror/lld/blob/master/ELF/ICF.cpp#L404

This patch adds a test case that triggers the mentioned code to execute.

llvm-svn: 335738