The first argument to an x86-interrupt ABI function was implicitly
treated as byval prior to LLVM 12. Since LLVM 12, it has to be
marked as such explicitly: llvm/llvm-project@2e0e03c

LLVM requires CMake 3.13.4, which is only available as of Ubuntu 20.04.
On images using an older version, build it manually.

Python 2 is needed for Clang 10, Python 3 for LLVM 12.

The Python 2 dependency could be removed by upgrading to Clang 11,
but that causes linker errors of unclear origin.

The definition of this struct changes in LLVM 12 due to the addition
of branch coverage support. To avoid future mismatches, declare our
own struct and then convert between them.

When cross-compiling to solaris/illumos targets, set
CMAKE_SYSTEM_NAME to SunOS.

musl-toolchain.sh is called with REPLACE_CC=1, so it will replace
the host compiler and the subsequent cmake build will fail because
it cannot find the openssl headers.

Move the cmake build earlier, so it happens before the compiler
is replaced.

This works around a design defect in the LLVM 12 pass builder
implementation. In LLVM 13, the PreLink ThinLTO pipeline properly
respects the OptimizerLastEPCallbacks.

We previously used -Os to disable this pass, but since
https://reviews.llvm.org/D91673 this no longer works. Explicitly
disable it using -Cllvm-args instead.