Kexec-tools purgatory/ broken on x86* when built with clang

[rxrbln]

When build with clang as default system compiler, kexec-tools fails to execute new kernel at least on x86. For now we workaround purgatory/ to build with gcc in r66036 but fixing the complex and partially real-mode code, assembler and linker trickery to also work with llvm/clang would be welcome.