Bumping to LLVM 12 with a new CMake

[dmikushin]

Hello, @tetzank thank you and to your coauthors for developing Coat. This PR bumps the used version of LLVM to 12, which requires to change just a couple of lines. More importantly, this PR revises the CMake build script in the following ways:

• LLVM 12 is to be provided externally (it's much easier for a user to obtain prebuilt LLVM binaries from distro, than to build them from source)
• asmjit is built "in-source" together with the project, so no extra script is needed
• Two flavors of coat header-only library are created in CMake as interface targets, and the corresponding flags and linked libraries are assigned to them (so, user applications, tests and examples no longer need to declare dependencies themselves)

[dmikushin]

One thing to note: LLVM 12 has compatibility issues with C++20 standard required by some debugging features (required keyword, etc.). Attempting to compile LLVM 12 with C++20 gives the following errors:

/usr/lib/llvm-12/include/llvm/ADT/STLExtras.h:1823:18: error: expected ‘)’ before ‘const’
 1823 |   result_pair<R>(const result_pair<R> &Other)

In this PR C++20 requirement is temporary downgraded to C++17.