target-mips: fix save_cpu_state() calls

The rule is:
- don't save PC if the exception is only triggered by softmmu.
- save PC if the exception can be triggered by an helper.

Fix a 64-bit kernel crash when loading modules.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
(cherry picked from commit b835e919)