wasi: add nonblock_test.go from gotip, fix nonblock read on Unix-like (#1517)

Signed-off-by: Edoardo Vacchi <evacchi@users.noreply.github.com>

internal/sysfs/file_unsupported.go

@@ -0,0 +1,12 @@
+//go:build !unix && !linux && !darwin
+
+package sysfs
+
+import "syscall"
+
+const NonBlockingFileIoSupported = false
+
+// readFd returns ENOSYS on unsupported platforms.
+func readFd(fd uintptr, buf []byte) (int, syscall.Errno) {
+	return -1, syscall.ENOSYS
+}