ggml-hexagon: fix swiglu failure at test-backend-ops part2

[chraac]

In PR #17344, we fixed some swiglu/silu ops but left hvx_fast_sigmoid_f32 unaddressed. This PR completes the fix by resolving the remaining failed swiglu/silu ops.

Changes

• Fix hvx_fast_sigmoid_f32 implementation to resolve remaining NaN and accuracy issues

Before

[SILU] NaN at index 231 (HTP0=-nan CPU=88.449669)   SILU(type=f32,ne_a=[128,2,2,2],v=0): �[1;31mFAIL�[0m
  SILU(type=f32,ne_a=[5,7,11,13],v=0): �[1;32mOK�[0m
[SWIGLU] NaN at index 122 (HTP0=nan CPU=-11446.431641)   SWIGLU(type=f32,ne_a=[128,2,2,2],v=0,swapped=0): �[1;31mFAIL�[0m
  SWIGLU(type=f32,ne_a=[5,7,11,13],v=0,swapped=0): �[1;32mOK�[0m
[SWIGLU] NMSE = 3.835742624 > 0.000000100   SWIGLU(type=f32,ne_a=[128,2,2,2],v=0,swapped=1): �[1;31mFAIL�[0m
  SWIGLU(type=f32,ne_a=[5,7,11,13],v=0,swapped=1): �[1;32mOK�[0m
[SWIGLU] NaN at index 216 (HTP0=nan CPU=-8444.154297)   SWIGLU(type=f32,ne_a=[128,2,2,2],v=0,split): �[1;31mFAIL�[0m
  SWIGLU(type=f32,ne_a=[5,7,11,13],v=0,split): �[1;32mOK�[0m

After

  SILU(type=f32,ne_a=[128,2,2,2],v=0): �[1;32mOK�[0m
  SILU(type=f32,ne_a=[5,7,11,13],v=0): �[1;32mOK�[0m
  SWIGLU(type=f32,ne_a=[128,2,2,2],v=0,swapped=0): �[1;32mOK�[0m
  SWIGLU(type=f32,ne_a=[5,7,11,13],v=0,swapped=0): �[1;32mOK�[0m
  SWIGLU(type=f32,ne_a=[128,2,2,2],v=0,swapped=1): �[1;32mOK�[0m
  SWIGLU(type=f32,ne_a=[5,7,11,13],v=0,swapped=1): �[1;32mOK�[0m
  SWIGLU(type=f32,ne_a=[128,2,2,2],v=0,split): �[1;32mOK�[0m
  SWIGLU(type=f32,ne_a=[5,7,11,13],v=0,split): �[1;32mOK�[0m