Main algorithm:
The Taylor series expansion of `asin(x)` is:
```math
\begin{align*}
asin(x) &= x + x^3 / 6 + 3x^5 / 40 + ... \\
&= x \cdot P(x^2) \\
&= x \cdot P(u) &\text{, where } u = x^2.
\end{align*}
```
For the fast path, we perform range reduction mod 1/64 and use degree-7
(minimax + Taylor) polynomials to approximate `P(x^2)`.
When `|x| >= 0.5`, we use the transformation:
```math
u = \frac{1 + x}{2}
```
and apply half-angle formula to reduce `asin(x)` to:
```math
\begin{align*}
asin(x) &= sign(x) \cdot \left( \frac{\pi}{2} - 2 \cdot asin(\sqrt{u}) \right) \\
&= sign(x) \cdot \left( \frac{\pi}{2} - 2 \cdot \sqrt{u} \cdot P(u) \right).
\end{align*}
```
Since `0.5 <= |x| <= 1`, `|u| <= 0.5`. So we can reuse the polynomial
evaluation of `P(u)` when `|x| < 0.5`.
For the accurate path, we redo the computations in 128-bit precision
with degree-15 (minimax + Taylor) polynomials to approximate `P(u)`.
This reverts commit bada9220b87e73c0f4a498b82f883e17eda928d1.
Revert "[libc][stdfix] Implement fixed point `countlsfx` functions in llvm-libc (#125356)"
This reverts commit f2a1103b323492160d7d27a1575fbda709b49036.
This commit does a few things:
* creates libc/docs/headers/ and moves all user API related headers under it.
* updates paths and docgen
* updates the top level index to put these headers under a new "Implementation
Status" tab.
* rename some of the files to be foo.rst for foo.h (except strings, which is
currently a mix of string.h and stdlib.h)
* update the heading of some files to be in the form foo.h.
