Support MIPS64EL relocation type names

MIPS64EL relocation entries have up to three relocation operations. Because
libObject only exposes a single relocation name, use the concatenation of
the individual relocation type names.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179357 91177308-0d34-0410-b5e6-96231b3b80d8