Add FS and VS bits to xpistatus

[christian-herber-nxp]

mstatus.{FS, VS} should be mirrored into xpistatus to assist context saving.
Additionally, the push instruction should set both VS and FS to 0 after saving the context, to ensure an ISR using the FPU or vector unit without saving the context prior (and setting these flags to clean) will cause an illegal instruction exception.
No modification needed for pop instruction.

[mark-honman]

Following the meeting discussion, my view is:
a. the general approach in the PR is good, i.e. save/restore of FS and VS
b. clearing of FS is a new behaviour and should not be enabled by default. However it is useful to make this configurable.

[christian-herber-nxp]

it is only a behavior of the push instruction, which anyway is a new instruction. Do you have any concerns with that?
It would not be breaking any existing code anyway.
Also, there is no sane code even if not existing which would be broken through this.