feat(uucore): support fs feature on more platforms

[xtqqczze]

The fs feature now compiles successfully on Hermit and WASI:

RUSTC_BOOTSTRAP=1 cargo check -q -Zbuild-std=std,panic_abort \
  -p uucore --features fs --all-targets \
  --target powerpc64-ibm-aix \
  --target x86_64-linux-android \
  --target x86_64-pc-cygwin \
  --target aarch64-apple-darwin \
  --target x86_64-unknown-freebsd \
  --target x86_64-unknown-haiku \
  --target x86_64-unknown-hermit \
  --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-pc-solaris \
  --target wasm32-wasip1 \
  --target x86_64-pc-windows-gnu

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/pr/bounded-memory (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/follow-name (passes in this run but fails in the 'main' branch)