[libc] fix build on aarch64 (#73739)

* avoid implicit narrowing conversion
* move hsearch entrypoints to FULL_BUILD

[libc] [search] implement hcreate(_r)/hsearch(_r)/hdestroy(_r) (#73469)

This patch implements `hcreate(_r)/hsearch(_r)/hdestroy(_r)` as
specified in https://man7.org/linux/man-pages/man3/hsearch.3.

[libc] [search] implement hcreate(_r)/hsearch(_r)/hdestroy(_r) (#73469)

This patch implements `hcreate(_r)/hsearch(_r)/hdestroy(_r)` as
specified in https://man7.org/linux/man-pages/man3/hsearch.3.html.

Notice that `neon/asimd` extension is not yet added in this patch.

- The implementation is largely simplified from rust's
[`hashbrown`](https://github.com/rust-lang/hashbrown/blob/master/src/raw/mod.rs)
as we only consider fix-sized insertion-only hashtables. Technical
details are provided in code comments.

- This patch also contains a portable string hash function, which is
derived from [`aHash`](https://github.com/tkaitchuck/aHash)'s fallback
routine. Not using any SIMD acceleration, it has a good enough quality
(passing all SMHasher tests) and is not too bad in speed.

- Some general functionalities are added, such as `memory_size`,
`offset_to`(alignment), `next_power_of_two`, `is_power_of_two`.
`ctz/clz` are extended to support shorter integers.

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