Enable `SPV_INTEL_fp_fast_math_mode` back

[anmyachev]

Need to revert: 38a1984

[whitneywhtsang]

16% performance regression on ATTN D_HEAD=128 CAUSAL=1 on BMG.

By relanding the change, the performance regression is recovered.
CI: https://github.com/intel/intel-xpu-backend-for-triton/actions/runs/15887596137/job/44803253499