[openmp] [elf_common] Fix linking against LLVM dylib

The hand-rolled linking logic in elf_common does not account for
the possibility of using LLVM dylib rather than a dozen static
libraries.  Since it does not seem to be easily convertible
to add_llvm_library, just hand-roll support for LLVM_LINK_LLVM_DYLIB.
This is necessary to support stand-alone builds against installed LLVM.

Differential Revision: https://reviews.llvm.org/D111038