[wasm] Make FileManager.createFile() work on WASI

fixes swiftwasm/swift#5593

`FileManager.createFile()` currently doesn't work on WASI because it
requires `.atomic`, it requires creating a temporary file, and it isn't
suppported on WASI.

So I have fixed that by removing the `.atomic` requirement only on WASI.

Sources/FoundationEssentials/FileManager/FileManager+Files.swift

@@ -276,6 +276,9 @@ extension _FileManagerImpl {
             }
             attr?[.protectionKey] = nil
         }
+        #elseif os(WASI)
+        // WASI doesn't support a temporary file, so can't use `.atomic`
+        let opts: Data.WritingOptions = []
         #else
         let opts = Data.WritingOptions.atomic
         #endif