[ELF] - Move the addition of synthetics from addPredefinedSections()

These were 3 last synthetics that were added in addPredefinedSections() instead
of createSyntheticSections(). Now it is possible to move addition to correct common place.

Also patch fixes testcase which discards .shstrtab, by restricting doing that.

Differential revision: https://reviews.llvm.org/D28561

llvm-svn: 291908