[LA64_DYNAREC] fix COMISS/UCOMISS flags state handling

[yzewei]

The reason for canceling DF is that using SF_SET_DF in the 0F 2E/2F path of LA64 dynarec causes single-precision SVD results from scipy.linalg to be incorrect. The corresponding double-precision COMISD/UCOMISD (66 0F 2E/2F) uses the immediate flags path (SF_SET), so it does not have the same problem.

Reproduce code:
You will need an x86 Python program and the relevant packages (scipy && numpy) installed.

#pip3 install numpy scipy
./box64 python3 - <<'PY'  import numpy as np
from scipy.linalg.lapack import sgesvd
A = np.array([[1.,2.,3.],[4.,5.,6.],[7.,8.,10.]], dtype=np.float32, order='F')
u, s, vt, info = sgesvd(A, compute_uv=1, full_matrices=1, overwrite_a=0)
print(s, info)
PY                                               

I suspect that arm64 also has a similar problem.dynarec_arm64_0f.c