[ELF] - Referencing __start or __stop should keep the section from GC.

This fixes the https://llvm.org/bugs/show_bug.cgi?id=22906 bug.

In GNU Binutils, a reference to start or stop is sufficient to 
prevent the section from being garbage collected.
Patch implements the same behavior for lld.

Differential revision: http://reviews.llvm.org/D17502

llvm-svn: 261840
6 files changed