OpenMP check fails during HIP device compile

Clang HIP (and probably CUDA) compilation have two different preprocessing passes, one for the device code and one for the host code. During the device preprocessing path `_OPENMP` isn't defined, which causes this check to fail:

https://github.com/LLNL/RAJA/blob/08cbbafd2d21589ebf341f7275c229412d0fe903/include/RAJA/config.hpp.in#L157-L165

As a consequence, the HIP and CPU OpenMP backends can't be built at the same time. If that check is sentineled somehow, possibly with a `defined(__HIP_DEVICE_COMPILE__) || defined(__CUDA_ARCH__)` the CPU OpenMP and HIP/CUDA-clang backends could both be built.

This smaller code showcases it a little more compactly:
``` omp_test.cpp
#include

#ifndef _OPENMP
#define _OPENMP 0
#warning "OpenMP Not Defined"
#endif

#ifndef __HIP_DEVICE_COMPILE__
#warning "Device compile not defined"
#endif

int main()
{
printf("OpenMP version %d\n", _OPENMP);
}
```

And at compilation (note the target for each warning)
```
$ hipcc -fopenmp -x hip omp_test.cpp
omp_test.cpp:5:2: warning: "OpenMP Not Defined" [-W#warnings]
#warning "OpenMP Not Defined"
^
1 warning generated when compiling for gfx803.
omp_test.cpp:9:2: warning: "Device compile not defined" [-W#warnings]
#warning "Device compile not defined"
^
1 warning generated when compiling for host.
```

Start at include/RAJA/config.hpp.in lines 157-165 and reproduce the issue with the omp_test.cpp example and the hipcc -fopenmp -x hip command. Check that the OpenMP configuration check handles the device and host preprocessing passes so the CPU OpenMP and HIP/CUDA-clang backends can build together.

Written by the indexing model from the issue text.