[RFC] Using mangle name DF16b for X86 __bf16 type

We used u6__bf16 as the mangle name of __bf16 when enable it on X86. GCC folks think it is better to use the same mangle name with the new C++ type std::bfloat16_t.
I noticed there are other targets that using u6__bf16 at the moment. Raise the question here to see if we can unify it in all targets.
I have put a patch for X86 to match with GCC: ⚙ D136919 [X86][RFC] Change mangle name of __bf16 from u6__bf16 to DF16b