Fix remove_dir_all on Fuchsia after CVE fix.

With the previous `is_dir` impl, we would attempt to unlink
a directory in the None branch, but Fuchsia supports returning
ENOTEMPTY from unlinkat() without the AT_REMOVEDIR flag because
we don't currently differentiate unlinking files and directories
by default.

On the Fuchsia side I've opened https://fxbug.dev/92273 to discuss
whether this is the correct behavior, but it doesn't seem like
addressing the error code is necessary to make our tests happy.

Updates std's libc crate to include DT_UNKNOWN for Fuchsia.

Author: anp

std/Cargo.toml

@@ -15,7 +15,7 @@ cfg-if = { version = "0.1.8", features = ['rustc-dep-of-std'] }
 panic_unwind = { path = "../panic_unwind", optional = true }
 panic_abort = { path = "../panic_abort" }
 core = { path = "../core" }
-libc = { version = "0.2.108", default-features = false, features = ['rustc-dep-of-std'] }
+libc = { version = "0.2.116", default-features = false, features = ['rustc-dep-of-std'] }
 compiler_builtins = { version = "0.1.66" }
 profiler_builtins = { path = "../profiler_builtins", optional = true }
 unwind = { path = "../unwind" }

std/src/sys/unix/fs.rs

@@ -1631,7 +1631,6 @@ mod remove_dir_impl {
         target_os = "illumos",
         target_os = "haiku",
         target_os = "vxworks",
-        target_os = "fuchsia"
     ))]
     fn is_dir(_ent: &DirEntry) -> Option<bool> {
         None
@@ -1642,7 +1641,6 @@ mod remove_dir_impl {
         target_os = "illumos",
         target_os = "haiku",
         target_os = "vxworks",
-        target_os = "fuchsia"
     )))]
     fn is_dir(ent: &DirEntry) -> Option<bool> {
         match ent.entry.d_type {