We use sigaltstack internally which on some systems is a syscall
and should be used as such. Move the x86-64 version to the Linux
specific directory and create in its place a file which always
causes compile errors.
If longjmp restores the stack frame to an address which is beyond
the stack frame at the time of the longjmp call it would install
an uninitialized stack frame. If compiled with _FORTIFY_SOURCE
defined, longjmp will now bail out in this situation.