Use Mode instead of mode_t in std::fs fields

[dtolnay]

Slightly different implementation of #122812 discussed in #122812 (review).

r? @ghost

[rust-log-analyzer]

The job x86_64-gnu-tools failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

test result: ok. 24 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.04s

     Running tests/ui.rs (obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/release/deps/ui-34e0c79da4e404df)
## Running ui tests in tests/pass for x86_64-unknown-linux-gnu
   Compiler: "MIRI_ENV_VAR_TEST"="0" "MIRI_TEMP"="/tmp/miri-uitest-BVHWaI" "RUST_BACKTRACE"="1" /checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/miri "--error-format=json" "-Dwarnings" "-Dunused" "-Ainternal_features" "-Zui-testing" "--target" "x86_64-unknown-linux-gnu" "--out-dir" OUT_DIR


   Compiler: "MIRI_ENV_VAR_TEST"="0" "MIRI_TEMP"="/tmp/miri-uitest-BVHWaI" "RUST_BACKTRACE"="1" /checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/miri "--error-format=json" "-Dwarnings" "-Dunused" "-Ainternal_features" "-Zui-testing" "--target" "x86_64-unknown-linux-gnu" "--out-dir" OUT_DIR

## Running ui tests in tests/panic for x86_64-unknown-linux-gnu

test result: ok. 27 passed; 4 ignored;
test result: ok. 27 passed; 4 ignored;

   Compiler: "MIRI_ENV_VAR_TEST"="0" "MIRI_TEMP"="/tmp/miri-uitest-BVHWaI" "RUST_BACKTRACE"="1" /checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/miri "--error-format=json" "-Dwarnings" "-Dunused" "-Ainternal_features" "-Zui-testing" "--target" "x86_64-unknown-linux-gnu" "--out-dir" OUT_DIR


   Compiler: "MIRI_ENV_VAR_TEST"="0" "MIRI_TEMP"="/tmp/miri-uitest-BVHWaI" "RUST_BACKTRACE"="1" /checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/miri "--error-format=json" "-Dwarnings" "-Dunused" "-Ainternal_features" "-Zui-testing" "--target" "x86_64-unknown-linux-gnu" "--out-dir" OUT_DIR


FAILED TEST: tests/fail/shims/fs/isolated_file.rs
FAILED TEST: tests/fail/shims/fs/isolated_file.rs
command: MIRI_ENV_VAR_TEST="0" MIRI_TEMP="/tmp/miri-uitest-BVHWaI" RUST_BACKTRACE="1" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/miri" "--error-format=json" "-Dwarnings" "-Dunused" "-Ainternal_features" "-Zui-testing" "--target" "x86_64-unknown-linux-gnu" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/ui/tests/fail/shims/fs" "tests/fail/shims/fs/isolated_file.rs" "--edition" "2021"
error: actual output differed from expected
Execute `./miri test --bless` to update `tests/fail/shims/fs/isolated_file.stderr` to the actual output
--- tests/fail/shims/fs/isolated_file.stderr
+++ <stderr output>
+++ <stderr output>
Error: 
   0: ui tests in tests/fail for x86_64-unknown-linux-gnu failed
   1: tests failed

Backtrace omitted. Run with RUST_BACKTRACE=1 environment variable to display it.
Run with RUST_BACKTRACE=full to include source snippets.
   --> RUSTLIB/std/src/sys/pal/PLATFORM/fs.rs:LL:CC
    |
-LL |         let fd = cvt_r(|| unsafe { open64(path.as_ptr(), flags, opts.mode as c_int) })?;
+LL |         let fd = cvt_r(|| unsafe { open64(path.as_ptr(), flags, opts.mode.0 as c_int) })?;
---
   = help: pass the flag `-Zmiri-disable-isolation` to disable isolation;
   = help: or pass `-Zmiri-isolation-error=warn` to configure Miri to return an error code from isolated operations (if supported for that operation) and continue with a warning
   = note: BACKTRACE:
   = note: inside closure at /checkout/library/std/src/sys/pal/unix/fs.rs:1140:36: 1140:86
   = note: inside `std::sys::pal::unix::cvt_r::<i32, {closure@std::sys::pal::unix::fs::File::open_c::{closure#0}}>` at /checkout/library/std/src/sys/pal/unix/mod.rs:321:19: 321:22
   = note: inside closure at /checkout/library/std/src/sys/pal/unix/fs.rs:1128:42: 1128:66
   = note: inside closure at /checkout/library/std/src/sys/pal/unix/fs.rs:1128:42: 1128:66
   = note: inside `std::sys::pal::common::small_c_string::run_with_cstr_stack::<std::sys::pal::unix::fs::File>` at /checkout/library/std/src/sys/pal/common/small_c_string.rs:49:18: 49:22
   = note: inside `std::sys::pal::common::small_c_string::run_with_cstr::<std::sys::pal::unix::fs::File>` at /checkout/library/std/src/sys/pal/common/small_c_string.rs:29:18: 29:47
   = note: inside `std::sys::pal::common::small_c_string::run_path_with_cstr::<std::sys::pal::unix::fs::File>` at /checkout/library/std/src/sys/pal/common/small_c_string.rs:19:5: 19:58
   = note: inside `std::fs::OpenOptions::_open` at /checkout/library/std/src/fs.rs:1135:9: 1135:42
   = note: inside `std::fs::OpenOptions::open::<&std::path::Path>` at /checkout/library/std/src/fs.rs:1131:9: 1131:34
   = note: inside `std::fs::File::open::<&str>` at /checkout/library/std/src/fs.rs:375:9: 375:58
note: inside `main`