require -fPIC on arm64 in addition to x86_64 and arm With gcc-8, a-c-c now fails to build its test cases with a relocation error on arm64. Add arm64 to the list of archs where we require -fPIC.
Run packing commands in a subprocess On low-memory VMs (such as autopkgtest runners at scale), a-c-c can OOM when trying to launch a subprocess towards the end of the run due to the main process's memory usage being >= 50% of available system memory. Since freeing memory for no-longer-needed variables is non-trivial in perl, just address this by creating a subprocess for handling any system() calls late in the process.
