[SYCL] Introduce backend macros

According to SYCL spec: backend macro should be specified for all supported
backends (defined in the Khronos group). It also refers to extension macros
(aka feature test macro) for other backends introduced by a vendor. Khronos
group defines only OpenCL backend and other backends supported by DPC++
should be treated as extensions.

The following macros are defined unconditionaly because backend support
is always available on DPC++ compile stage:
 - OpenCL backed:      SYCL_BACKEND_OPENCL
 - Level Zero backend: SYCL_EXT_ONEAPI_BACKEND_LEVEL_ZERO
 - CUDA backend:       SYCL_EXT_ONEAPI_BACKEND_CUDA
 - ESIMD_CPU backend:  SYCL_EXT_INTEL_BACKEND_ESIMD_CPU
 - HIP backend:        SYCL_EXT_ONEAPI_BACKEND_HIP
Extra libs may be required to link DPC++ application and run it.

Author: vladimirlaz

sycl/include/CL/sycl/backend/opencl.hpp

@@ -16,6 +16,7 @@
 #include <CL/sycl/detail/cl.h>
 #include <CL/sycl/kernel_bundle.hpp>
 
+#define SYCL_BACKEND_OPENCL 1
 __SYCL_INLINE_NAMESPACE(cl) {
 namespace sycl {
 

sycl/include/CL/sycl/feature_test.hpp

@@ -29,6 +29,9 @@ namespace sycl {
 #define SYCL_EXT_INTEL_MEM_CHANNEL_PROPERTY 1
 #define SYCL_EXT_INTEL_USM_ADDRESS_SPACES 1
 #define SYCL_EXT_ONEAPI_BACKEND_LEVEL_ZERO 1
+#define SYCL_EXT_ONEAPI_BACKEND_CUDA 1
+#define SYCL_EXT_INTEL_BACKEND_ESIMD_CPU 1
+#define SYCL_EXT_ONEAPI_BACKEND_HIP 1
 
 } // namespace sycl
 } // __SYCL_INLINE_NAMESPACE(cl)