uucore: support fsext feature on more platforms #14933

RUSTC_BOOTSTRAP=1 cargo clippy -q -Zbuild-std=std,panic_abort -p uucore --features=fsext --all-targets \
  --target=powerpc64-ibm-aix \
  --target=i686-linux-android \
  --target=x86_64-pc-cygwin \
  --target=aarch64-apple-darwin \
  --target=x86_64-unknown-dragonfly \
  --target=x86_64-unknown-freebsd \
  --target=x86_64-unknown-haiku \
  --target=x86_64-unknown-hurd-gnu \
  --target=aarch64-unknown-illumos \
  --target=x86_64-unknown-linux-gnu \
  --target=i686-unknown-netbsd \
  --target=x86_64-unknown-openbsd \
  --target=x86_64-unknown-redox \
  --target=x86_64-pc-solaris \
  --target=wasm32-wasip1 \
  --target=x86_64-pc-windows-gnu