Avoid creating symbol table entries for registers

Avoid creating symbol table entries for registers

Instructions like "jal t0, foo" were erroneously creating symbol table
entries for t0 as well as foo, which causes linking problems.  Fix by
reordering instruction alternatives so that t0 is first attempted to
be parsed as a register, rather than as a symbol.

	* riscv-opc.c (riscv_opcodes): Reorder jal and call entries.