Dedicated Third Party Targets in Project Tree

[JoGei]

This PR is open to discussion. The ideas and motivation are as follows:

• (1) There is a lot of legacy third party code "hiding" down the project tree of ETISS, e.g., pugixml

• (2) Some third party code is fetched and built in CMakeLists.txt which makes it hard to maintain, e.g., the external softfloat in the JIT library libsoftfloat

• (3) Current approach, for ELFIO and SimpleIni, does fetch(+build) in the top level CMakeLists.txt, which bloats the top level CMake description (but is IMO better than (1) and (2).

Introducing a dedicated third party node in the project source tree could help establish a "how to integrate" foreign code. My proposed solution works for CMake-FetchContent, CMake-ExternalProject, and Git-Submodules, should we ever decide to use those instead. The core idea is to establish CMake targets in the new third_party node that are added before e.g. the src directory is added. This should make those targets available to multiple targets - be it ETISS, Arch or JIT.

[github-actions]

Performance Statistics

Status for the gcc Just-In-Time Engine (for commit 7c0e4a5): No significant performance change
Current dhrystone MIPS for gcc JIT : 95.07
Previous best for gcc JIT (recorded in commit 4d2d26f): 108.31, difference -12.22%

Status for the llvm Just-In-Time Engine (for commit 7c0e4a5): Regressed since commit 78716e7
Current dhrystone MIPS for llvm JIT : 24.11
Previous best for llvm JIT (recorded in commit 940dbdd): 36.11, difference -33.23%

Status for the tcc Just-In-Time Engine (for commit 7c0e4a5): No significant performance change
Current dhrystone MIPS for tcc JIT : 61.93
Previous best for tcc JIT (recorded in commit 40c0b92): 62.38, difference -0.71%

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