Similar to previous migration done for other targets (PowerPC, X86, ARM, etc). Switch from the confusing VariantKind to Specifier, which aligns with Arm and IBM AIX's documentation. In addition, rename *MCExpr::getKind, which confusingly shadows the base class getKind. In the future, relocation specifiers should be encoded as part of SystemZMCExpr instead of MCSymbolRefExpr.