[libc] Make alloca visible from stdlib.h (#241695)

Let `<stdlib.h>` also declare `alloca`. Some code assumes that it'll
be exposed through `<stdlib.h>` header, plus `<alloca.h>` is not
a standard header anyway and is missing on some platforms (e.g.
on BSD systems). Use YAML fragments, similar to what we do
for non-standard `<malloc.h>` header on GNU systems.

AI disclosure: this patch was written with Claude Code. I reviewed the
change and built it in a full build, and the include and header tests
pass

Assisted-by: Claude
GitOrigin-RevId: 0a12975a45526e0aea74c644107d85cd7dedb8a5