LoongArch: ld: Fix bug not generate plt when link a dso

  Fix the bug that can not generate func@plt
  when linking a undefined function with cmodel=medium.
  Add testcase.

  bfd/
    * elfnn-loongarch.c
  ld/testsuite/ld-loongarch-elf/
    * cmodel-libjirl.dd
    * cmodel.exp
    * libjirl.s