🐛 [rtl] fix MEPC value for instruction access faults (#458)

* [sw/lib] RTE: minor optimizations

* [sw/example] modify i-access exception test

for instruction fetch access exceptions MEPC has to point to the address that caused the exception (i.e. the failing instruction fetch itself)

* [docs] add warning regarding non-resumable exceptions

* [CHANGELOG] add v1.7.8.8

* [sw/example] processor_check: minor cleanup

* 🐛 [rtl] fix xEPC value on instruction access fault

* [rtl] minor edit

add missing default level dcsr.cause CSR