Support file `alloc-checker` is missing from cmake

There's no cmake target for `libc/src/__support/alloc-checker.h`. It should have one and that new target should be a dependency for the various uses in our codebase.

Start by locating the CMake definitions for libc and the references to libc/src/__support/alloc-checker.h. Add the missing CMake target and verify that the various code uses depend on it, then configure or build the relevant libc targets to confirm the dependency is recognized.

Written by the indexing model from the issue text.