[core] support .gnu.linkonce sections

.gnu.linkonce sections are similar to section groups. They were supported before
section groups existed and provided a way to resolve COMDAT sections using a
different design. There are few implementations that use .gnu.linkonce sections
to store simple floating point constants which doesnot require complex section
group support but need a way to store only one copy of the floating point
constant. .gnu.linkonce based symbol resolution achieves that.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@205163 91177308-0d34-0410-b5e6-96231b3b80d8