[libc] Add compound assignment operator overloads for BFloat16 (#201301)
The current Bfloat16 has normal operator overloads `+` , `-` , `=`,
`!=`, `*`, & `/`.
Later during a function failure `*=` was added in
https://github.com/llvm/llvm-project/pull/182882
For completeness the rest of the operators: `/=`, `+=`, `-=` are added
These are added along with some smoke test .