gas/RISC-V: adjust assembler for opcode table re-ordering

PR gas/29940

With the single-operand JAL entry now sitting ahead of the two-operand
one, the parsing of a two-operand insn would first try to parse an 'a'-
style operand, resulting in the insertion of bogus (and otherwise
unused) undefined symbols in the symbol table, having register names.
Since 'a' is used as 1st operand only with J and JAL, and since JAL is
the only insn _also_ allowing for a register as 1st operand (and then
there being a 2nd one), special case this parsing aspect right there.

Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com>