[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 in a binary.

.gnu.linkonce based symbol resolution achieves that.

Review : http://llvm-reviews.chandlerc.com/D3242

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@205280 91177308-0d34-0410-b5e6-96231b3b80d8