Support breakpoint intrinsic
The `breakpoint` intrinsic raises a `SIGTRAP` signal. If a debugger is attached to a normal program, then `SIGTRAP` can be used to trigger breakpoints in debuggers like `gdb`. If there is no debugger, then the program exits with a message like `Trace/breakpoint trap (core dumped)`. This adds support for the intrinsic in Miri. While actually passing through the `SIGTRAP` doesn't make sense in a Miri context (if it just raised the signal normally then it would allow for debugging Miri itself, not the program being evaluated). As such, it just raises an error.
Add atomic min and max
Closes#1718
Previous attempt: #1653
TODO:
- [x] Merge `atomic_op` and `atomic_min_max` functions
- [x] Fix CI
**Note:** this PR also removes arbitrary trailing whitespace and generally formats the affected files
add date to Zulip notification subject
We sometimes have discussions in those threads; let's make sure we have a new thread for each new day (which likely means a new problem... or nobody got around to fixing things yet^^).
adjust CI test flags
`RUST_TEST_NOCAPTURE` seems to make no difference except for making compiletest output not use colors any more; `RUST_BACKTRACE` is already set by `compiletest.rs`.
rustup; fix tests for new MIR optimization
Somehow https://github.com/rust-lang/rust/pull/78360 manages to mask UB. This would make sense if there were loops or things like that, but there are not, so really this is just very confusing...
fix reborrowing of tagged ZST references
`@SkiFire13` [pointed out](https://github.com/rust-lang/rust/pull/82554#issuecomment-787088712) that Miri fails to detect illegal use of empty slices. This PR fixes that. In so doing, it uncovers a flaw of Stacked Borrows: it is incompatible with how the formatting machinery uses `extern type`, so for now we skip reborrowing when we cannot determine the exact size of the pointee.
