Port r292146.
    
    Give priority to linker scripts over preemption.
    
    LLD exports symbols that are also present in used shared libraries to
    make sure they are preempted at runtime. That is a reasonable default,
    but we must allow for it to be overwritten with linker script. If we
    don't, libraries that expect to be able to hide a c++ delete operator
    will fail.
    
    This should fix the firebird build.
    
    llvm-svn: 292370