System.Native: Make configure.cmake checks work for iOS (dotnet/corefx#42408)

System.Native: Make configure.cmake checks work for iOS (dotnet/corefx#42408)

Apple platforms like macOS/iOS allow targeting older operating system versions with a single SDK,
the mere presence of a symbol in the SDK doesn't tell us whether the deployment target really supports it.
The compiler raises a warning when using an unsupported API, turn that into an error so `check_symbol_exists()`
can correctly identify whether the API is supported on the target.

This fixes compile errors after https://github.com/dotnet/corefx/pull/37583 was merged since `clonefile()` is iOS10+.

Replace usages of `check_function_exists()` since it only tests for existence of functions and the CMake docs
recommend using `check_symbol_exists()`: https://cmake.org/cmake/help/v3.15/module/CheckFunctionExists.html

Commit migrated from https://github.com/dotnet/corefx/commit/879127eb87346200b4ddf40a7fce96e79ec211fe