[wasm] Fix cross-compilation on macOS host machine

We didn't set the CMAKE_SYSTEM_NAME and CMAKE_SYSTEM_PROCESSOR when
cross-compiling stdlib for WASI. This caused the build to fail on macOS
host machines as the build system was trying to build some overlays
assuming the target is macOS.
Additionally, add a default "SWIFT_HOST_VARIANT_SDK" for WASI target
even though we don't support it as a host system yet because it's
required anyway.

Author: kateinoigakukun

CMakeLists.txt

@@ -155,6 +155,8 @@ else()
     set(SWIFT_HOST_VARIANT_SDK_default "ANDROID")
   elseif("${CMAKE_SYSTEM_NAME}" STREQUAL "Darwin")
     set(SWIFT_HOST_VARIANT_SDK_default "OSX")
+  elseif("${CMAKE_SYSTEM_NAME}" STREQUAL "WASI")
+    set(SWIFT_HOST_VARIANT_SDK_default "WASI")
   else()
     message(FATAL_ERROR "Unable to detect SDK for host system: ${CMAKE_SYSTEM_NAME}")
   endif()

utils/swift_build_support/swift_build_support/products/wasmstdlib.py

@@ -44,6 +44,8 @@ def build(self, host_target):
     def _build(self, host_target, target_triple):
         self.cmake_options.define('CMAKE_INSTALL_PREFIX:PATH', '/usr')
         self.cmake_options.define('CMAKE_BUILD_TYPE:STRING', self._build_variant)
+        self.cmake_options.define('CMAKE_SYSTEM_NAME:STRING', 'WASI')
+        self.cmake_options.define('CMAKE_SYSTEM_PROCESSOR:STRING', 'wasm32')
         self.cmake_options.define(
             'SWIFT_STDLIB_BUILD_TYPE:STRING', self._build_variant)