[CPU][IBM Z] Fix BF16 support and vectorize math operations for s390x (VXE)

- Fix BF16 byte ordering for big-endian architecture
- Vectorize exp(), tanh(), erf() functions with polynomial approximations
- Add FMA intrinsics (fma, fms, nfma, nfms) using vec_madd/vec_msub
- Improve BF16 rounding with round-to-nearest-even
- Fix prefetch implementation
- Add sigmoid, gelu_tanh, gelu_erf, rcp, rsqrt operations
- Implement softmax_fp32vec8 and rmsnorm_fp32vec8 kernels
- Fix FP16 support by aliasing to FP32Vec16
- Exclude s390x from FP16 vector trait in cpu_attn_impl.hpp

Signed-off-by: Rehan Khan <[email protected]>

Author: R3hankhan123

csrc/cpu/cpu_attn_impl.hpp

@@ -841,7 +841,7 @@ struct VecTypeTrait<c10::BFloat16> {
 };
 #endif
 
-#if !defined(__powerpc__)
+#if !defined(__powerpc__) && !defined(__s390x__)
 template <>
 struct VecTypeTrait<c10::Half> {
   using vec_t = vec_op::FP16Vec16;