This PR fixes broken links in all files describing libc usage modes.
Please let me know if there are any other places that need updating.

---------

Co-authored-by: shubhp@perlmutter <shubhp@perlmutter.com>

Docgen updates the docs when the config options are changed. This update
has been waiting since https://github.com/llvm/llvm-project/pull/143187.

Reverts llvm/llvm-project#143810 

This PR breaks our buildbot:
https://lab.llvm.org/buildbot/#/builders/10/builds/7159 revert to
unblock downstream merge.

In #143802 the stdio test cleanup missed a few places where errno was
being set to a failing value, and one where the framework needed to
included.

This is the first step in preparation for:
https://discourse.llvm.org/t/rfc-make-clang-builtin-math-functions-constexpr-with-llvm-libc-to-support-c-23-constexpr-math-functions/86450

Add `CLOCKS_PER_SEC` and the older `CLK_TCK`. Allows the user to define
a `__CLK_TCK` to override if necessary.

Also add an extra column for embedded AArch64 in `time.rst`

- 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}
```

Fix typos in GPU libc's documentation about the rpc implementation.