Fixes a couple of bugs found when building. The PR to enable the headers
can be found here: #144114.

- math.yaml: float128 guard
- wchar.yaml: __restrict keyword order

- Implementation of atan (tan inverse) function for 16-bit inputs.
- Exhaustive tests across the 16-bit input range

We reduce computation of `acos` to `asin` as follow:

When `|x| < 0.5`:
```math
acos(x) = \frac{\pi}{2} - asin(x).
```
For `0.5 <= |x| < 1`, let
```math
u = \frac{1 - \left| x \right|}{2},
```
then
```math
acos(x) = \begin{cases}
  2 \cdot asin \left( \sqrt{u} \right) &, 0.5 \leq x < 1 \\
  \pi - 2 \cdot asin \left( \sqrt{u} \right) &, -1 < x \leq 0.5 
\end{cases}
```

Implementation of atanhf16 function for 16-bit inputs.

Closes: https://github.com/llvm/llvm-project/issues/132209

Addresses #132211  #132754
Part of #95250

Part of https://github.com/llvm/llvm-project/issues/131642.

Implement hypot for Float16 along with tests.

Implement asinh for Float16 along with tests. Closes #131001

Implementation of fmaf16 function for 16-bit inputs.

Implementation of acoshf16 function for 16-bit inputs.