libpdbg/p8chip.c: Emulate sreset using ramming for active threads
libpdbg/p8chip.c: Emulate sreset using ramming for active threads

Based on patch from Alistair, some fixes and changes:
- account HILE bit, set/clear MSR_LE
- clear MSR_PR
- don't use raw ramming (clearer this way, not perf critical)

At the moment, must manually stop all threads in the core, and manually
restart them. Can change behaviour depending on what exactly we want
(e.g., sreset all threads may be good for debugging).

open-power/pdbg,open-power/pdbg,open-power/pdbg