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.