Rollup merge of #56979 - VardhanThigle:Vardhan/rust-sgx-unwind-support, r=alexcrichton

Adding unwinding support for x86_64_fortanix_unknown_sgx target.

Unwinding support is provided by our port of LLVM's libunwind which is available from https://github.com/fortanix/libunwind/tree/release_50.

libunwind requires support for rwlock and printing to stderr, which is only provided by `std` for this target. This poses two problems: 1) how to expose the `std` functionality to C and 2) dependency inversion.

### Exposing `std`

For exposing the functionality we chose to expose the following symbols:

* __rust_rwlock_rdlock
* __rust_rwlock_wrlock
* __rust_rwlock_unlock
* __rust_print_err
* __rust_abort

Also, the following are needed from `alloc`:

* __rust_alloc
* __rust_dealloc

#### Rust RWLock in C

In `libunwind`, RWLock is initialized as a templated static variable:

```c
pthread_rwlock_t DwarfFDECache<A>::_lock = PTHREAD_RWLOCK_INITIALIZER;
```

I don't know of a good way to use the Rust sys::rwlock::RWLock type and initializer there. We could have a static global variable in Rust, but that doesn't work with the templating. The variable needs to be initialized statically, since this target doesn't support the .init section. Currently, I just used a byte array and standard C array initialization. The mapping between this C type and the Rust type needs to be manually maintained. There is a compile-time check and a unit test to make sure the Rust versions of these C definitions match the actual Rust type. If any reviewer knows of a better solution, please do tell.

### Dependency inversion issue

`std` depends on `panic_unwind` which depends on `libunwind`, and `libunwind` depends on `std`. This is not normally supported by Rust's linking system. Therefore we use raw C exports from `std` *and* `libunwind.a` is linked last in the target `post_link_objects` instead of being built as part of the Rust `libunwind`. Currently, all C exports are defined in `src/libstd/sys/sgx/rwlock.rs` to overcome LTO issues. Only the `__rust_rwlock_*` definitions *need* to live there for privacy reasons. Once again, if any reviewer knows of a better solution, please do tell.

r? @alexcrichton

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