wasm: Build static SDK overlay explicitly

The static SDK overlay has been built implicitly for a long time, but
this is not the case anymore after 5bf2c93.
We need to build it explicitly now.

Author: kateinoigakukun

utils/swift_build_support/swift_build_support/products/wasmstdlib.py

@@ -87,6 +87,7 @@ def _build(self, host_target, target_triple):
         # Build only static stdlib
         self.cmake_options.define('SWIFT_BUILD_STATIC_STDLIB:BOOL', 'TRUE')
         self.cmake_options.define('SWIFT_BUILD_DYNAMIC_STDLIB:BOOL', 'FALSE')
+        self.cmake_options.define('SWIFT_BUILD_STATIC_SDK_OVERLAY:BOOL', 'TRUE')
         self.cmake_options.define('SWIFT_STDLIB_TRACING:BOOL', 'FALSE')
         self.cmake_options.define('SWIFT_STDLIB_HAS_ASLR:BOOL', 'FALSE')
         self.cmake_options.define('SWIFT_STDLIB_CONCURRENCY_TRACING:BOOL', 'FALSE')