Allow FS to be imprecise

[palmer-dabbelt]

I can't find anything in the spec that explicitly allows FS to be imprecise. This table lists the possible FS state transitions:

riscv-isa-manual/src/machine.tex

Line 824 in 5f8294b

Table~\ref{fsxsstates} shows all the possible state transitions for

There's some vague "When the status is Dirty, the corresponding state has potentially been modified since the last context save.", but the state transition table makes it appear that it'l not legal to set the dirty state without executing an instruction.

[aswaterman]

@palmer-dabbelt i still agree this is a problem, but i'm going to punt it back to you to propose a PR.