[ELF][MIPS] Revert r264761 and add test case to demonstrate the problem

If we make R_MIPS_LO16 a relative relocation, linker:
- never creates R_MIPS_COPY relocation for it
- attempts to create R_MIPS_REL32 dynamic relocation if R_MIPS_LO16's
  target is a preemptible symbol

Differential Revision: http://reviews.llvm.org/D18607

llvm-svn: 264956