target/riscv: Fix manually parsed 16 bit insn

target/riscv: Fix manually parsed 16 bit insn

during the refactor to decodetree we removed the manual decoding that is
necessary for c.jal/c.addiw and removed the translation of c.flw/c.ld
and c.fsw/c.sd. This reintroduces the manual parsing and the
omited implementation.

Signed-off-by: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
Tested-by: Palmer Dabbelt <palmer@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Tested-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>