[mips] Implement missing relocations in the integrated assembler.

%got_hi, %got_lo, %call_hi, %call_lo, %higher, and %highest are now recognised
by MipsAsmParser::getVariantKind().

To prevent future issues with missing entries in this StringSwitch, I've added
an assertion to the default case.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205200 91177308-0d34-0410-b5e6-96231b3b80d8