2454: Restore QNX8 jobs r=japaric a=Hoverbear

Reverses #2440

Co-authored-by: Ana Hobden <[email protected]>

2422: update QNX scripts to work with QEMU built from source r=jyn514 a=japaric

`qemu-system-$ARCH -netdev bridge,(..)` uses the qemu-bridge-helper script that was installed with `qemu-system-$ARCH`. However, to set up the bridge network in the QNX scripts, we are using the qemu-bridge-helper of the system installed QEMU.

When using a QEMU built from source, we have seen the VM fail to initialize due to a different version of qemu-bridge-helper being used to set up the network bridge. To ensure the right qemu-bridge-helper is using in the QNX script, we search for it using the absolute path of the `qemu-system-$ARCH` as the reference, instead of assuming it'll be in `/usr` as that only works when the system installed QEMU is used.

2439: QNX target renaming r=jyn514 a=japaric

**NOTE** depends and includes PR #2454 

This is a rebased version of #2418 now that rust-lang/rust#158449 has been merged. the first six commits in this PR are a cherry picks the commits in rust-lang/rust#158449. the rest of commits are a cherry pick of #2418. note that commit [fix rustdoc-html tests](1483680) has been dropped because it became unnecessary with upstream commit rust-lang/rust@4eec7f5

---

- `aarch64-unknown-nto-qnx800` becomes simply `aarch64-unknown-qnx`
- `x86_64-pc-nto-qnx800` becomes simply `x86_64-pc-qnx`
- references to QNX OS and QNX Neutrino RTOS are replaced with QNX SDP (Software Development Platform) for uniformity
- various `nto_qnx` modules are renamed to `qnx_sdp` to match the above

The new target names are more consistent with those used by the QNX SDP 8.0 toolchain, and reflect a level on ongoing API stability similar to that for macOS and Linux (which simply have minimum supported versions record for their targets). The new target names also match what the copy of `clang` shipped in the QNX Desktop Image (part of QNX SDP 8.0) self reports as (that is, `clang` running self-hosted *on* QNX).

2448: Remove gcc submodule and mark GCC codegen backend as unsupported r=fs-rachel a=fs-rachel



2451: Sync dependencies between linkchecker and dist tarball r=jyn514 a=fs-rachel



Co-authored-by: Jorge Aparicio <[email protected]>
Co-authored-by: Jonathan Pallant <[email protected]>
Co-authored-by: Rachel Barker <[email protected]>