riscv: simplify backtrace report

* We already have a header 'backtrace', there is no need to repeat the
  word backtrace on every line.
* Add a blank line before the backtrace section of the crash report for
  improved readability.
* If U-Boot is compiled without backtrace, there is no need to write a
  message at all.
* Avoid #ifdef. We prefer functions to always be compiled and let
  the linker remove them if not needed.
* Foresee 3 digits for the backtrace index.

For testing the 'exception' command can be used.

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