When the interpreter calls MSRUpdated, we should update the membase
variable. Not because the interpreter itself needs it, but because the
JIT needs it if it's falling back to the interpreter for an instruction
that sets the MSR.
Additionally, the JIT's FallBackToInterpreter needs to read back the new
membase value afterwards.
This fixes games crashing on JitArm64 if mtmsr is set to fall back to
interpreter. I was unable to reproduce the issue on Jit64, presumably
due to a fortunate series of coincidences (instructions that set MSR are
always followed by an exception exit, and
PowerPCManager::CheckExternalExceptions was always calling
JitInterface::UpdateMembase, and Jit64::WriteExceptionExit was always
calling Jit64::EmitUpdateMembase.)