[lld][elf] Add --dynamic-linker option to the ELF linker.
Users can override the default value of the dynamic linker to be set to the
one that appears in the command line. The path can even be empty!. 

Added a test for the option.


git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@182889 91177308-0d34-0410-b5e6-96231b3b80d8