283: fix segfaulting rustc when `download-ci-llvm = true` r=tshepang a=japaric

- the segfault occurred in the second / third C++ frame after `rustc` called into LLVM using the `PassWrapper.cpp` glue
- the segfault occurred due a ABI mismatch between those two frames
- both frames corresponded to LLVM C++ functions. (the first frame was the `PassWrapper.cpp` glue)
- one of the frames (functions) came from the LLVM library built in CI; the other frame was codegen'd locally
- when comparing to a rust-lang/rust build, it was found that both frames involved in the segfault were present in the LLVM library built in rust-lang/rust CI. the rust-lang/rust artifact did not result in a segfault
- the differences between the rust-lang/rust and ferrocene build settings are: (a) rust-lang/rust builds llvm as shared library; ferrocene builds them as static libraries. (b) rust-lang/rust uses `clang` and a newer version of `gcc` / `g++`
- building LLVM as shared libraries in ferrocene did not fix the segfault but additionally bumping the `gcc` / `g++` version (by way of using a newer base docker image) did

Co-authored-by: Jorge Aparicio <jorge.aparicio@ferrous-systems.com>

283: fix segfaulting rustc when `download-ci-llvm = true` r=tshepang a=japaric

- the segfault occurred in the second / third C++ frame after `rustc` called into LLVM using the `PassWrapper.cpp` glue
- the segfault occurred due a ABI mismatch between those two frames
- both frames corresponded to LLVM C++ functions. (the first frame was the `PassWrapper.cpp` glue)
- one of the frames (functions) came from the LLVM library built in CI; the other frame was codegen'd locally
- when comparing to a rust-lang/rust build, it was found that both frames involved in the segfault were present in the LLVM library built in rust-lang/rust CI. the rust-lang/rust artifact did not result in a segfault
- the differences between the rust-lang/rust and ferrocene build settings are: (a) rust-lang/rust builds llvm as shared library; ferrocene builds them as static libraries. (b) rust-lang/rust uses `clang` and a newer version of `gcc` / `g++`
- building LLVM as shared libraries in ferrocene did not fix the segfault but additionally bumping the `gcc` / `g++` version (by way of using a newer base docker image) did

Co-authored-by: Jorge Aparicio <jorge.aparicio@ferrous-systems.com>

371: Properly exclude `build-metrics.json` from Tar r=tshepang a=Hoverbear

Plucked from #283, this fixes the "Invocation Stacking" issue in the generated reports from CI.

Co-authored-by: Jorge Aparicio <jorge.aparicio@ferrous-systems.com>

376: Update to Ubuntu 20.04 and fix downloading LLVM from CI r=pvdrz a=pietroalbini

This PR is based on #283 and #362: it updates CI to run on Ubuntu 20.04 (fixing the failure in #305), and changes how we build LLVM to hopefully fix `download-ci-llvm`. Notably, this bumps the minimum glibc version requirement to 2.31.

The hard part of this PR was fixing the broken CI for Aarch64. It turns out that the failure was caused by a breaking change in `binfmt-support` package, which now requires manually enabling new binfmts after importing them (if `/proc/sys/fs/binfmt_misc` is not mounted, like in CI).

Co-authored-by: Jorge Aparicio <jorge.aparicio@ferrous-systems.com>
Co-authored-by: Tshepang Mbambo <tshepang.mbambo@ferrous-systems.com>
Co-authored-by: Pietro Albini <pietro.albini@ferrous-systems.com>