`-fsycl` breaks `_OPENMP` preprocessor definition

**Describe the bug**
Compiling a program which makes use of `_OPENMP` to check for the OpenMP version will result in a broken macro if `-fsycl` is used as a compilation flag.

**To Reproduce**
```c++
#include

#if _OPENMP < 200203
#error Oops!
#endif

auto main() -> int
{
std::cout << "OpenMP version: " << _OPENMP << std::endl;
}
```

Without SYCL:

```
icpx -std=c++17 -fiopenmp
OpenMP version: 201811
```

With SYCL:

```
icpx -std=c++17 -fsycl -fiopenmp
:4:2: error: Oops!
#error Oops!
^
:9:40: error: use of undeclared identifier '_OPENMP'
std::cout << "OpenMP version: " << _OPENMP << std::endl;
^
2 errors generated.
Compiler returned: 1
```

See it [here](https://godbolt.org/z/4j671jT1v) on godbolt.