Add a test to check that NVCC is used for CUDA and Clang is used for pure host compilation

[andportnoy]

XLA has tests built-in to help check that:

bazel build //build_tools/configure:assert_nvcc //build_tools/configure:assert_clang

[andportnoy]

More context, from JAX docs:

While it should typically be possible to compile jaxlib from source using most modern compilers, the builds are only tested using clang. Pull requests are welcomed to improve support for different toolchains, but other compilers are not actively supported.

In the past we had issues when we accidentally used GCC to build, but thought we were using Clang.
//build_tools/configure:assert_clang tests that the compiler used to compile C++ under Bazel is Clang.