HIP: Add CMAKE_HIP_PLATFORM variable to specify GPU platform

For now, require the value to be `amd`, since that is the only
platform we currently support.

Help/manual/cmake-variables.7.rst

@@ -590,6 +590,7 @@ Variables for Languages
    /variable/CMAKE_Fortran_MODOUT_FLAG
    /variable/CMAKE_HIP_ARCHITECTURES
    /variable/CMAKE_HIP_EXTENSIONS
+   /variable/CMAKE_HIP_PLATFORM
    /variable/CMAKE_HIP_STANDARD
    /variable/CMAKE_HIP_STANDARD_REQUIRED
    /variable/CMAKE_ISPC_HEADER_DIRECTORY

Help/prop_tgt/HIP_ARCHITECTURES.rst

@@ -3,7 +3,8 @@ HIP_ARCHITECTURES
 
 .. versionadded:: 3.21
 
-List of AMD GPU architectures to generate device code for.
+List of GPU architectures to for which to generate device code.
+Architecture names are interpreted based on :variable:`CMAKE_HIP_PLATFORM`.
 
 A non-empty false value (e.g. ``OFF``) disables adding architectures.
 This is intended to support packagers and rare cases where full control

Help/release/dev/hip-nvidia.rst

@@ -0,0 +1,6 @@
+hip-nvidia
+----------
+
+* The :variable:`CMAKE_HIP_PLATFORM` variable was added to specify
+  the GPU platform for which HIP language sources are to be compiled
+  (``amd``).

Help/variable/CMAKE_HIP_ARCHITECTURES.rst

@@ -3,10 +3,14 @@ CMAKE_HIP_ARCHITECTURES
 
 .. versionadded:: 3.21
 
-Default value for :prop_tgt:`HIP_ARCHITECTURES` property of targets.
+List of GPU architectures to for which to generate device code.
+Architecture names are interpreted based on :variable:`CMAKE_HIP_PLATFORM`.
 
-This is initialized to the architectures reported by ``rocm_agent_enumerator``,
-if available, and otherwise to the default chosen by the compiler.
+This is initialized based on the value of :variable:`CMAKE_HIP_PLATFORM`:
+
+``amd``
+  Uses architectures reported by ``rocm_agent_enumerator``, if available,
+  and otherwise to a default chosen by the compiler.
 
 This variable is used to initialize the :prop_tgt:`HIP_ARCHITECTURES` property
 on all targets. See the target property for additional information.

Help/variable/CMAKE_HIP_PLATFORM.rst

@@ -0,0 +1,19 @@
+CMAKE_HIP_PLATFORM
+------------------
+
+.. versionadded:: 3.28
+
+GPU platform for which HIP language sources are to be compiled.
+
+The value must be one of:
+
+``amd``
+  AMD GPUs
+
+If not specified, the default is ``amd``.
+
+:variable:`CMAKE_HIP_ARCHITECTURES` entries are interpreted with
+as architectures of the GPU platform.
+
+:variable:`CMAKE_HIP_COMPILER <CMAKE_<LANG>_COMPILER>` must target
+the same GPU platform.