WASI: Add platform modules

Add minimal platform modules to support toolchain files like those in

* https://github.com/WebAssembly/wasi-sdk

or direct use of `-DCMAKE_SYSTEM_NAME="WASI"`

Fixes: #19223

Help/manual/cmake-variables.7.rst

@@ -346,6 +346,7 @@ Variables that Describe the System
    /variable/MSVC_VERSION
    /variable/MSYS
    /variable/UNIX
+   /variable/WASI
    /variable/WIN32
    /variable/WINCE
    /variable/WINDOWS_PHONE

Help/variable/CMAKE_SYSTEM_NAME.rst

@@ -85,6 +85,7 @@ Value                     Name
 ``UNIX_SV``               SCO UnixWare (pre release 7)
 ``UnixWare``              SCO UnixWare 7
 ``visionOS``              Apple mixed reality operating system
+``WASI``                  WebAssembly System Interface
 ``watchOS``               Apple watch operating system
 ``Windows``               Windows stationary operating systems
 ``WindowsCE``             Windows Embedded Compact

Help/variable/WASI.rst

@@ -0,0 +1,7 @@
+WASI
+----
+
+.. versionadded:: 3.31
+
+Set to ``1`` when the target system is WebAssembly System Interface
+(:variable:`CMAKE_SYSTEM_NAME` is ``WASI``).

Modules/Platform/WASI-Initialize.cmake

@@ -0,0 +1 @@
+set(WASI 1)

Modules/Platform/WASI.cmake

@@ -0,0 +1 @@
+# WASI Platform