SystemInformation: Fix compilation on GNU/{kFreeBSD,Hurd}

Includes of cxxabi.h, execinfo.h, etc. were guarded more restrictively (e.g. additionally by __linux) than the code which needs them (just by e.g. KWSYS_SYSTEMINFORMATION_HAS_CPP_DEMANGLE). kFreeGNU has some of these headers so extend the __linux ifdef to kFreeBSD and Hurd as well.

Ideally the code should be fixed to have the same level of ifdef protection for both include and use of these APIs, but this is good enough for now pending further investigation.

This patch is taken from the debian CMake 3.13.3-2 package patch.