Rewrite description of FCSR.

[kito-cheng]

Try to rewrite description of FCSR, remove "thread storage duration", and reference of specific language standard, and also try to describe properties for different fields.

Description of frm are borrowed some word from the C lang spec[1].
And fflags might be changed by floating point operations or any function call, and no any promise of the value on the function entry.
Description of other bits are reference unpriv spec.

[1] ISO/IEC 9899:2011: 7.6.3

— a function call does not alter its caller’s floating-point control modes, clear its caller’s
floating-point status flags, nor depend on the state of its caller’s floating-point status
flags unless the function is so documented;

[kito-cheng]

@nick-knight I don't know why I can add @aswaterman to reviewer but can't add you, so tag you manually :P

[nick-knight]

@kito-cheng: I don't have privilege to review PRs in riscv-non-isa repos like this one. An admin will have to extend my privileges. (I would argue that this privilege should be a default, but I don't fully understand the ramifications.)