[Fuzzer] Enable custom libc++ for Android (#70407)

The Android LLVM build system builds the arm64 fuzzer lib without
HWASan, but then applications that enable HWASan can generated an object
file with a HWASan-ified version of some libc++ symbols (e.g.
`std::__1::piecewise_construct`). The linker can choose the HWASan-ified
definition, but then it cannot resolve the relocation from
libclang_rt.fuzzer-aarch64-android.a to this symbol because the high
bits of the address are unexpectedly set. This produces an error:

```
relocation R_AARCH64_ADR_PREL_PG_HI21 out of range
```

Fix this problem by linking a custom isolated libc++ into Android's
fuzzer library.

We need to pass through ANDROID_NATIVE_API_LEVEL so that the libc++ for
32-bit Android (API < 24) uses LLVM_FORCE_SMALLFILE_FOR_ANDROID.