When panic != unwind, `nounwind` is added to all functions for a target.
This can cause issues when a panic happens with RUST_BACKTRACE=1, as
there needs to be a way to reconstruct the backtrace. There are three
possible sources of this information: forcing frame pointers (for which
an option exists already), debug info (for which an option exists), or
unwind tables.

Especially for embedded devices, forcing frame pointers can have code
size overheads (RISC-V sees ~10% overheads, ARM sees ~2-3% overheads).
In code, it can be the case that debug info is not kept, so it is useful
to provide this third option, unwind tables, that users can use to
reconstruct the call stack. Reconstructing this stack is harder than
with frame pointers, but it is still possible.

This commit adds a compiler option which allows a user to force the
addition of unwind tables. Unwind tables cannot be disabled on targets
that require them for correctness, or when using `-C panic=unwind`.

Follow-up from rust-lang#71716 I wasn't able to add in time.

…na-kruppe

Add Option to Force Unwind Tables

When panic != unwind, `nounwind` is added to all functions for a target.
This can cause issues when a panic happens with RUST_BACKTRACE=1, as
there needs to be a way to reconstruct the backtrace. There are three
possible sources of this information: forcing frame pointers (for which
an option exists already), debug info (for which an option exists), or
unwind tables.

Especially for embedded devices, forcing frame pointers can have code
size overheads (RISC-V sees ~10% overheads, ARM sees ~2-3% overheads).
In production code, it can be the case that debug info is not kept, so it is useful
to provide this third option, unwind tables, that users can use to
reconstruct the call stack. Reconstructing this stack is harder than
with frame pointers, but it is still possible.

---

This came up in discussion on rust-lang#69890, and turned out to be a fairly simple addition.

r? @hanna-kruppe

…ulacrum

Remove clippy from some leftover lists of "possibly failing" tools

rust-lang#70655 successfully made clippy get built and tested on CI on every merge, but the lack of emitted toolstate info caused the toolstate to get updated to test-fail. We should remove clippy entirely from toolstate, as it now is always test-pass.

The changes made in this PR reflect what we do for `rustdoc`, which is our preexisting tool that is gated on CI.

r? @Mark-Simulacrum

…ul, r=estebank

Suggest removing semicolon in last expression only if it's type is known

Fixes rust-lang#67971

Is there a syntax for explicitly checking if a note doesn't exist in test output? Something like `//~ !NOTE ...`

I believe r? @estebank deals with diagnostics.

…nnethercote

Improve docs for embed-bitcode and linker-plugin-lto

Follow-up from rust-lang#71716 I wasn't able to add in time.