@l and friends adjust their value depending the context used in.

@l and friends adjust their value depending the context used in.
For ori, they are unsigned, for addi, signed. Create a new target
expression type to handle this and evaluate Fixups accordingly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215315 91177308-0d34-0410-b5e6-96231b3b80d8