cmd/exception: support RISC-V compressed instruction

Eliminating the C extension on application processors is under
discussion.

Support emitting a compressed instruction. This will lead to an
illegal instruction exception if the C extension is not implemented.

For testing build qemu-riscv64_defconfig with CONFIG_RISCV_ISA_C=n
and run with

    qemu-system-riscv64 -M virt -bios u-boot -nographic -cpu rv64,c=false

    => exception compressed
    Unhandled exception: Illegal instruction
    EPC: 0000000087731708 RA: 000000008773fe44 TVAL: 0000000000004501
    EPC: 000000008001b708 RA: 0000000080029e44 reloc adjusted

    Code: 0b93 0000 0493 0000 0993 0000 f06f ccdf (4501)

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>