MCValue: Change getAccessVariant to return uint16_t
Some targets encode the relocation specifier within SymA using
MCSymbolRefExpr::SubclassData. They will cast the specifier
to *MCExpr::Specifier.
Migrate away from the confusing MCSymbolRefExpr::VariantKind.
Note: getAccessVariant is a deprecated method to get the relocation
specifier.
