The number of exception for risc-v is 16 (0 ~ 15)
for the machine ISA version 1.12 or earlier, the number of exception is 20
(0 ~ 19) from the ISA version 1.13. And maybe changed in the future.
Using a dedicated option to control the exception number to allow the earlier
version chip with customized exception number (e.g. 16 ~ 19 used) to define
the exception reason string correctly.
Signed-off-by: Huang Qi <huangqi3@xiaomi.com>