PPC: Remove env->hreset_excp_prefix

PPC: Remove env->hreset_excp_prefix

This value is not needed if we use correctly the MSR[IP] bit.

excp_prefix is always 0x00000000, except when the MSR[IP] bit is
implemented and set to 1, in that case excp_prefix is 0xfff00000.

The handling of MSR[IP] was already implemented but not used at reset
because the value of env->msr was changed "manually".

The patch uses the function hreg_store_msr() to set env->msr, this
ensures a good handling of MSR[IP] at reset, and therefore a good value
for excp_prefix.

Signed-off-by: Fabien Chouteau <chouteau@adacore.com>
Signed-off-by: Alexander Graf <agraf@suse.de>