Nanosleep
This PR adds a shim for `libc::nanosleep`, (available under -Zmiri-disable-isolation only) which backs `thread::sleep` on Linux and macOS. I started off by extracting the `timespec` parsing from the `pthread_cond_timedwait` shim into a helper method, and adding checks for invalid values. The second commit adds the new shim and a small test. The shim blocks the current thread, and registers a timeout callback to unblock the thread again, using the same method as `pthread_cond_timedwait` does.
Move panic payload state from Machine to Thread
This PR moves the panic payload storage from the `Machine` state to per-thread state. Prior to this change, if one thread panicked while another was still unwinding, Miri would fail with `thread 'rustc' panicked at 'the panic runtime should avoid double-panics', src/shims/panic.rs:51:9`. I ran into this issue while prototyping a round-robin scheduler, but it's also reachable with the current scheduler and contrived programs that use blocking API calls to cause thread switching during unwinding. I wrote a test case along those lines for this change.
Re-organize platform-specific shims
Move platform-specific code to `shims::{posix::{linux, macos}, windows}`. Also make it private in these modules to ensure we are reasonably structured.
Add file sync shims
This PR adds shim implementations for these related file syncing functions.
* `fsync`, for POSIX targets, backed by `File::sync_all()`
* `fdatasync`, for POSIX targets, backed by `File::sync_data()`
* `fcntl` with command `F_FULLFSYNC`, for macOS targets, backed by `File::sync_all()`
* `sync_file_range`, for Linux targets, backed by `File::sync_data()`
