Doesn't build with FLEN != XLEN

[Timmmm]

When FLEN != XLEN there are some compilation issues in the fmv D instructions. Current code is:

function clause execute (F_UN_TYPE_D(rs1, rd, FMV_X_D)) = {
  assert(sizeof(xlen) >= 64);
  let rs1_val_D            = F(rs1)[63..0];
  let rd_val_X  : xlenbits = sign_extend(rs1_val_D);
  X(rd) = rd_val_X;
  RETIRE_SUCCESS
}

On RV64F the sizeof(xlen) >= 64) assertion passes, but then F(rs1)[63..0] can't compile because F(rs1) is only 32 bits. The fix is to add an assertion for flen too, since this function is only available when xlen==64 & flen==64.

Also the functions have a crazy level of intermediate variables and explicit type annotation. I'm using these much simpler versions:

function clause execute (F_UN_TYPE_S(rs1, rd, FMV_X_W)) = {
  // A narrower n-bit transfer out of the floating-point registers will
  // transfer the lower n bits of the register ignoring the upper FLEN-n bits.
  X(rd) = sign_extend(F(rs1)[31 .. 0]);
  RETIRE_SUCCESS
}

function clause execute (F_UN_TYPE_S(rs1, rd, FMV_W_X)) = {
  // A narrower n-bit transfer, n<FLEN, into the f registers will create a valid
  // NaN-boxed value.
  F(rd) = nan_box(X(rs1)[31..0]);
  RETIRE_SUCCESS
}

function clause execute (F_UN_TYPE_D(rs1, rd, FMV_X_D)) = {
  assert(sizeof(xlen) >= 64 & sizeof(flen) >= 64);
  X(rd) = F(rs1);
  RETIRE_SUCCESS
}

function clause execute (F_UN_TYPE_D(rs1, rd, FMV_D_X)) = {
  assert(sizeof(xlen) >= 64 & sizeof(flen) >= 64);
  F(rd) = X(rs1);
  RETIRE_SUCCESS
}

The spec for these for XLEN != FLEN is really hard to understand so it is nice to have concise Sail definitions.