RISC-V: adjust logic to avoid register name symbols

Special casing GPR names in my_getSmallExpression() leads to a number of
inconsistencies. Generalize this by utilizing the md_parse_name() hook,
limited to when instruction operands are being parsed (really: probed).
Then both the GPR lookup there and the yet more ad hoc workaround for
PR/gas 29940 can be removed (including its extension needed for making
the compressed form JAL work again).