Make Book-E debug register state part of the PCB context.

Previously, DBCR0 flags were set "globally", but this leads to problems
because Book-E fine grained debug settings work only in conjuction with the
debug master enable bit in MSR: in scenarios when the DBCR0 was set with
intention to debug one process, but another one with MSR[DE] set got
scheduled, the latter would immediately cause debug exceptions to occur upon
execution of its code instructions.

To avoid such problems and properly handle debugging context, DBCR0 state
should be managed individually per process.