riscv: show code leading to exception

To make analyzing exceptions easier output the code that leads to it.
We already do the same on the ARM platform.

Here is an example:

    => exception ebreak
    Unhandled exception: Breakpoint
    EPC: 000000008ff5d50e RA: 000000008ff5d62c TVAL: 0000000000000000
    EPC: 000000008020b50e RA: 000000008020b62c reloc adjusted

    Code: 2785 0693 07a0 dce3 fef6 47a5 d563 00e7 (9002)

To disassemble the code we can use the decodecode script:

    $ echo 'Code: 2785 0693 07a0 dce3 fef6 47a5 d563 00e7 (9002)' | \
      CROSS_COMPILE=riscv64-linux-gnu- scripts/decodecode

    Code: 2785 0693 07a0 dce3 fef6 47a5 d563 00e7 (9002)
    All code
    ========
       0:   2785                    addiw   a5,a5,1
       2:   07a00693                li      a3,122
       6:   fef6dce3                bge     a3,a5,0xfffffffffffffffe
       a:   47a5                    li      a5,9
       c:   00e7d563                bge     a5,a4,0x16
      10:*  9002                    ebreak         <-- trapping instruction
            ...

    Code starting with the faulting instruction
    ===========================================
       0:   9002                    ebreak
            ...

As it is not always clear if the first 16 bits are at the start or in the
middle of a 32bit instruction it may become necessary to strip the first
u16 from the output before calling decodecode to get the correct
disassembled code.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Sean Anderson <seanga2@gmail.com>

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