CUDA: Fix CUDA_ARCHITECTURES policy CMP0104 check in try_compile

Since commit e98588aaba (CUDA: Add CUDA_ARCHITECTURES target property,
2020-03-12), `CMAKE_CUDA_ARCHITECTURES` is both initialized and required
by policy CMP0104's `NEW` behavior.  Inside a `try_compile` project we
use `cmake_minimum_required(VERSION ...)` with the current running
version of CMake.  Now that our version number is 3.18, `CMP0104` gets
set to `NEW` in the `try_compile`.  If the outer projects does not set
the policy then `CMAKE_CUDA_ARCHITECTURES` is not available and the
`try_compile` fails to generate.

Teach `try_compile` to check the outer project's policy status and
set policy CMP0104 to `OLD` if needed to match.

Fixes: #20794

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