[compiler-rt] [Fuzzer] Enable custom libc++ for Android (PR #70407)

[Ryan Prichard via llvm-commits]

rprichard wrote:

The fuzzer's custom libc++ is built with `-DCMAKE_TRY_COMPILE_TARGET_TYPE=STATIC_LIBRARY`, so the `LIBCXX_HAS_PTHREAD_LIB` etc feature testing in `libcxx/cmake/config-ix.cmake` doesn't work. It decides that the `rt` and `pthread` libaries *do* exist, and outputs `#pragma comment(lib, "...")` directives that break linking.

It looks like I can pass the appropriate variables manually, but we also have OS-specific sections in libcxx and libcxxabi, so I'll add sections for Android and upload a separate PR for that.

https://github.com/llvm/llvm-project/pull/70407