[WasmFS] Simplify read/write in the Node backend. NFC (#26595)

In line with commit c77be74.

Split out from #24733.

Author: kleisauke

src/lib/libwasmfs_node.js

@@ -206,7 +206,7 @@ addToLibrary({
     return wasmfsTry(() => {
       // TODO: Cache open file descriptors to guarantee that opened files will
       // still exist when we try to access them.
-      let nread = fs.readSync(fd, new Int8Array(HEAPU8.buffer, buf_p, len), 0, len, pos);
+      let nread = fs.readSync(fd, HEAPU8, buf_p, len, pos);
       {{{ makeSetValue('nread_p', 0, 'nread', 'i32') }}};
       // implicitly return 0
     });
@@ -217,7 +217,7 @@ addToLibrary({
     return wasmfsTry(() => {
       // TODO: Cache open file descriptors to guarantee that opened files will
       // still exist when we try to access them.
-      let nwritten = fs.writeSync(fd, new Int8Array(HEAPU8.buffer, buf_p, len), 0, len, pos);
+      let nwritten = fs.writeSync(fd, HEAPU8, buf_p, len, pos);
       {{{ makeSetValue('nwritten_p', 0, 'nwritten', 'i32') }}};
       // implicitly return 0
     });