riscv: add backtrace support

riscv: add backtrace support

When debugging, it is useful to have a backtrace to find
out what is in the call stack as the previous function (RA)
may not have been the culprit.

Since this adds size to the build, do not add it by default
and avoid putting it in the SPL build if not needed.

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Tested-by: Leo Yu-Chi Liang <ycliang@andestech.com>