Allow for read only 0 FS and VS bits in mstatus

[qbojj]

The spec allows for mstatus.FS to be read only 0 if F extension is not enabled, but sail does not currently support the behaviour.
Similar clause would be for mstatus.VS.

[nadime15]

There is an open PR for this #1413. Similar PR is #1679 (merged), which handles the case where neither F/V nor S-mode is implemented