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.
Debug info refering to a TLS variable via DW_OP_GNU_push_tls_address
needs to use a R_390_TLS_LDO64 relocation instead of R_390_64.
Fixed by adding a SystemZELFTargetObjectFile override class and proving
a getDebugThreadLocalSymbol implementation.
