The main cause of the problem is that libstd/os/mod.rs treats emscripten targets as an alias of linux targets, whereas liblibc treats emscripten targets as musl-compliant, so it gets a slightly different struct stat64 defined.
This commit adds conditional compilation checks to use the correct timestamp format on fs metadata functions in the case of compiling to emscripten targets.

This commit also depends needs https://github.com/ashleysommer/rust/commit/f1575cff2d631e977038fdba3fa3422ba5f8f2fe applied in order to successfully build libstd with emscripten target.

Squashed 10 commits:
1) The main cause of the problem is that libstd/os/mod.rs treats emscripten targets as an alias of linux targets, whereas liblibc treats emscripten targets as musl-compliant, so it gets a slightly different struct stat64 defined.
This commit adds conditional compilation checks to use the correct timestamp format on fs metadata functions in the case of compiling to emscripten targets.

2) Update previous commit to comply with rust formatting standards.
Removed tab characters, remove trailing whitespaces.

3) Move emscripten changes into their own file under libstd/os/emscripten
Put libstd/os/linux/fs back to the way it was.

4) Cannot use stat.st_ctim on emscripten to get created time.

5) Remove compile-time conditionals for target_env = musl, it looks like musl builds compile fine already.

6) Undone some formatting changes that are no longer needed,
Removed some more target_env="musl" compilation checks that I missed from my previous commit.

7) upgrade to liblibc e19309c, it fixes the differences in the musl stat and stat64 definitions.

8) Undo the compile-time checks to check for emscripten (or musl targets) in the FileAttr struct.
No longer needed after updating liblibc to e19309c.

9) Change the MetadataExt implementation of emscripten fs.rs module to match the changes in new liblibc.

10) remove a stray return statement, should have been removed in the previous commit.

- make sure we copy the third party objects (crt*.o) to the target stage directory.
- apply the x86_64-musl logic also to the i686-musl target.

I have these tagged as stable in 1.9, so this shouldn't merge until the 1.8 beta's cut.

This commit is the result of the FCPs ending for the 1.8 release cycle for both
the libs and the lang suteams. The full list of changes are:

Stabilized

* `braced_empty_structs`
* `augmented_assignments`
* `str::encode_utf16` - renamed from `utf16_units`
* `str::EncodeUtf16` - renamed from `Utf16Units`
* `Ref::map`
* `RefMut::map`
* `ptr::drop_in_place`
* `time::Instant`
* `time::SystemTime`
* `{Instant,SystemTime}::now`
* `{Instant,SystemTime}::duration_since` - renamed from `duration_from_earlier`
* `{Instant,SystemTime}::elapsed`
* Various `Add`/`Sub` impls for `Time` and `SystemTime`
* `SystemTimeError`
* `SystemTimeError::duration`
* Various impls for `SystemTimeError`
* `UNIX_EPOCH`
* `ops::{Add,Sub,Mul,Div,Rem,BitAnd,BitOr,BitXor,Shl,Shr}Assign`

Deprecated

* Scoped TLS (the `scoped_thread_local!` macro)
* `Ref::filter_map`
* `RefMut::filter_map`
* `RwLockReadGuard::map`
* `RwLockWriteGuard::map`
* `Condvar::wait_timeout_with`

Closes #27714
Closes #27715
Closes #27746
Closes #27748
Closes #27908
Closes #29866

Also back out keepalive support for TCP since the API is perhaps not
actually what we want. You can't read the interval on Windows, and
we should probably separate the functionality of turning keepalive on
and overriding the interval.