[lld][ELF] Assign sectionChoice properly to ELF atoms

This sets the sectionChoice property for DefinedAtoms. The output section name
is derived by the property of the atom. This also decreases native file size.

Adds a test.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@190840 91177308-0d34-0410-b5e6-96231b3b80d8