[Mips] Do not hard-code the paired relocation type

Request `getPairRelocation()` function to get paired relocation type.
That allows us to look up another pairs like R_MICROMIPS_HI16/LO16
in the future.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@221539 91177308-0d34-0410-b5e6-96231b3b80d8