[libc] Create a guard for Bfloat16 type

Currently, due to the ABI difference, we are not able to test our Bfloat16 functions with CORE-MATH.
An effort to check whether this was the issue was done in https://github.com/llvm/llvm-project/pull/214966. Thus,
Once we add a guard for BFloat16, we would be able to work on and modify all the bfloat16 functions to be compatible with CORE-MATH for testing, if necessary.

Reproduce:
```sh
CORE_MATH_STD_NAME=hypotbf16 CORE_MATH_CHECK_STD=true EXTRA_CFLAGS='-DCORE_MATH_NOCHECK_UNDERFLOW=1' LIBM=~/llvm-project/build/libc/lib/libllvmlibc.a ./check.sh hypot_bf16
```