riscv: reset after crash

If an exception occurs on ARM or x86, we call panic() which will try to
reset the board. Do the same on RISC-V.

To avoid -Werror=format-zero-length move a '\n' to the string passed to
panic. We don't need a message here as depending on CONFIG_PANIC_HANG we
will either see

    ### ERROR ### Please RESET the board ###

or

    resetting ...

as next message.

Reviewed-by: Rick Chen <rick@andestech.com>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>