[ELF] - Do not use mergeable sections when LS is used.

After latest changes we combine input sections with 
different attributes into single output section. 
Problem here is that regular output sections does not
support adding mergeable input sections (and vise versa).
Patch just temporarily disables merging for now at 
the same way we do for -O0 for example.

This change helps for linking FreeBSD kernel.

Differential revision: https://reviews.llvm.org/D23447

llvm-svn: 278555