[ELF] - Linkerscripts: ignore CONSTRUCTORS in output section declaration.

It is used in linux kernel script:
http://lxr.free-electrons.com/source/arch/x86/kernel/vmlinux.lds.S#L140

Though CONSTRUCTORS is ignored for ELF.

Differential revision: https://reviews.llvm.org/D28951

llvm-svn: 292777