Embedded Wasm: enable `import WASILibc` (#83792)
WASILibc wasn't built for the embedded stdlib in `stdlib/public/Platform/CMakeLists.txt`. New `copy_wasilibc_modulemap_embedded_resource` and `embedded-stdlib-platform-${triple}` targets are added, the latter for `wasm32-unknown-wasip1` only for now. Also added a `wasilibc_functions.swift` test to verify the result.
Resolves https://github.com/swiftlang/swift/issues/83513
rdar://157467412
