interp: Handle abs/neg on nan values better.

`fabsf` was altering other bits.

Core/MIPS/MIPSIntVFPU.cpp

@@ -122,7 +122,7 @@ void ApplyPrefixST(float *r, u32 data, VectorSize size, float invalid = 0.0f)
 		{
 			r[i] = origV[regnum];
 			if (abs)
-				r[i] = fabs(r[i]);
+				((u32 *)r)[i] = ((u32 *)r)[i] & 0x7FFFFFFF;
 		}
 		else
 		{
@@ -130,7 +130,7 @@ void ApplyPrefixST(float *r, u32 data, VectorSize size, float invalid = 0.0f)
 		}
 
 		if (negate)
-			r[i] = -r[i];
+			((u32 *)r)[i] = ((u32 *)r)[i] ^ 0x80000000;
 	}
 }