we're executing on; besides dealing with the bits not implemented in the
601's MSR it also removes the silent failure behaviour when passing
PSL_VEC set on a CPU not implementing it.
Also, fix those masks for the 4xx again.
Add a PSL_USEROK_P(psl) macro which valids the bits (replaces the use of
PSL_USERSTATIC).
Add a PSL_USERSRR1 mask which is used to mask out status bits in the upper
half of SRR1.
Make sure PSL_VEC is set appropriately in userret(). PSL_VEC is in the same
region as SSR1 status bits so it's not preserved on exceptions. Thus we
need to make to set it.
When returning a MSR/SRR1 to userland, always clear the status bits.
Add emulation of the mfpvr, mtmsr, and mfmsr instructions.
