Include `count` in AppleClang toolchains.

The motivation here is so we can run the compiler-rt tests
from a standalone build against AppleClang.

In particular the `Posix/halt_on_error-torture.cpp` and
`Posix/halt_on_error_suppress_equal_pcs.cpp` ASan test cases currently
require this binary for the tests to pass.

rdar://76366784

Differential Revision: https://reviews.llvm.org/D100087