Allow using the SYCL execution space on AMD GPUs (kokkos#6321)

* Allow using the sycl execution space on AMD GPUs

Intended for testing purposes only and hence requires the
UNSUPPORTED_ARCH flag to be set (prefer the HIP execution
space for these GPUs).

* Fix arch warning messages

* Fix trailing whitespaces

cmake/kokkos_arch.cmake

@@ -89,7 +89,7 @@ KOKKOS_ARCH_OPTION(AMPERE86        GPU  "NVIDIA Ampere generation CC 8.6"  "KOKK
 KOKKOS_ARCH_OPTION(ADA89           GPU  "NVIDIA Ada generation CC 8.9"     "KOKKOS_SHOW_CUDA_ARCHS")
 KOKKOS_ARCH_OPTION(HOPPER90        GPU  "NVIDIA Hopper generation CC 9.0"  "KOKKOS_SHOW_CUDA_ARCHS")
 
-IF(Kokkos_ENABLE_HIP OR Kokkos_ENABLE_OPENMPTARGET OR Kokkos_ENABLE_OPENACC)
+IF(Kokkos_ENABLE_HIP OR Kokkos_ENABLE_OPENMPTARGET OR Kokkos_ENABLE_OPENACC OR Kokkos_ENABLE_SYCL)
   SET(KOKKOS_SHOW_HIP_ARCHS ON)
 ENDIF()
 
@@ -573,7 +573,7 @@ FUNCTION(CHECK_CUDA_ARCH ARCH FLAG)
     ENDIF()
     SET(CUDA_ARCH_ALREADY_SPECIFIED ${ARCH} PARENT_SCOPE)
     IF (NOT KOKKOS_ENABLE_CUDA AND NOT KOKKOS_ENABLE_OPENMPTARGET AND NOT KOKKOS_ENABLE_SYCL AND NOT KOKKOS_ENABLE_OPENACC)
-      MESSAGE(WARNING "Given CUDA arch ${ARCH}, but Kokkos_ENABLE_CUDA, Kokkos_ENABLE_OPENACC, and Kokkos_ENABLE_OPENMPTARGET are OFF. Option will be ignored.")
+      MESSAGE(WARNING "Given CUDA arch ${ARCH}, but Kokkos_ENABLE_CUDA, Kokkos_ENABLE_SYCL, Kokkos_ENABLE_OPENACC, and Kokkos_ENABLE_OPENMPTARGET are OFF. Option will be ignored.")
       UNSET(KOKKOS_ARCH_${ARCH} PARENT_SCOPE)
     ELSE()
       IF(KOKKOS_ENABLE_CUDA)
@@ -631,8 +631,8 @@ FUNCTION(CHECK_AMDGPU_ARCH ARCH FLAG)
       MESSAGE(FATAL_ERROR "Multiple GPU architectures given! Already have ${AMDGPU_ARCH_ALREADY_SPECIFIED}, but trying to add ${ARCH}. If you are re-running CMake, try clearing the cache and running again.")
     ENDIF()
     SET(AMDGPU_ARCH_ALREADY_SPECIFIED ${ARCH} PARENT_SCOPE)
-    IF (NOT KOKKOS_ENABLE_HIP AND NOT KOKKOS_ENABLE_OPENMPTARGET AND NOT KOKKOS_ENABLE_OPENACC)
-      MESSAGE(WARNING "Given AMD GPU architecture ${ARCH}, but Kokkos_ENABLE_HIP, Kokkos_ENABLE_OPENACC, and Kokkos_ENABLE_OPENMPTARGET are OFF. Option will be ignored.")
+    IF (NOT KOKKOS_ENABLE_HIP AND NOT KOKKOS_ENABLE_OPENMPTARGET AND NOT KOKKOS_ENABLE_OPENACC AND NOT KOKKOS_ENABLE_SYCL)
+      MESSAGE(WARNING "Given AMD GPU architecture ${ARCH}, but Kokkos_ENABLE_HIP, Kokkos_ENABLE_SYCL, Kokkos_ENABLE_OPENACC, and Kokkos_ENABLE_OPENMPTARGET are OFF. Option will be ignored.")
       UNSET(KOKKOS_ARCH_${ARCH} PARENT_SCOPE)
     ELSE()
       IF(KOKKOS_ENABLE_HIP)
@@ -772,6 +772,14 @@ IF (KOKKOS_ENABLE_SYCL)
     ELSE()
       MESSAGE(SEND_ERROR "Setting a CUDA architecture for SYCL is only allowed with Kokkos_ENABLE_UNSUPPORTED_ARCHS=ON!")
     ENDIF()
+  ELSEIF(AMDGPU_ARCH_ALREADY_SPECIFIED)
+    IF(KOKKOS_ENABLE_UNSUPPORTED_ARCHS)
+      COMPILER_SPECIFIC_FLAGS(
+        DEFAULT -fsycl-targets=amdgcn-amd-amdhsa -Xsycl-target-backend --offload-arch=${KOKKOS_AMDGPU_ARCH_FLAG}
+      )
+    ELSE()
+      MESSAGE(SEND_ERROR "Setting a AMDGPU architecture for SYCL is only allowed with Kokkos_ENABLE_UNSUPPORTED_ARCHS=ON!")
+    ENDIF()
   ELSEIF(KOKKOS_ARCH_INTEL_GEN)
     COMPILER_SPECIFIC_FLAGS(
       DEFAULT -fsycl-targets=spir64