[SYCL][CUDA][HIP] Define backend macros based on compiler

[npmiller]

Currently the backend macros are only defined when the runtime adapters for the given backend are built.

It's more accurate to use whether or not the compiler can target these backends, regardless of whether the runtime elements are present.

It should be possible to compile an application for a backend without having to install the matching runtime.

[againull]

@npmiller Do you know what might be the reason of this failure on CUDA:

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Timed Out Tests (1):
SYCL :: Basic/profile_host_task.cpp

I wasn't able to find any other references to this test or failures on other PRs. I'm not sure if something has changed, like drivers update on our CUDA runners.

[npmiller]

I can't reproduce this timeout locally and this test isn't using any interop or building any kernels so I don't see how this patch could affect it.

Looking at other PRs this one also has a suspicious unrelated timeout: #15021

Timed Out Tests (1):
  SYCL :: USM/alloc_functions.cpp

Even though the PR only touches this test: sycl/test-e2e/Matrix/get_coord_int8_matB.cpp, but that timeout is different from the one on this PR.

[againull]

SYCL :: Basic/profile_host_task.cpp

Ok, thank you for trying to reproduce locally.
I created #15025 for bookkeeping, if this failure reoccurs then we can investigate more thoroughly.