forkfd: clone() not available on Linux/ia64

[pinotree]

PR #94 introduces the usage of clone() in the Linux implementation of forkfd; that syscall is not available on the ia64 architecture, and Qt 6.4.0 (which includes the forkfd implementation from this repository) fails to build on ia64 because of that:

/usr/bin/ld: src/corelib/CMakeFiles/Core.dir/io/forkfd_qt.cpp.o: in function `system_vforkfd(int, int*, int (*)(void*), void*, int*)':
./obj-ia64-linux-gnu/./src/corelib/io/../../3rdparty/forkfd/forkfd_linux.c:171: undefined reference to `clone'
collect2: error: ld returned 1 exit status

https://buildd.debian.org/status/fetch.php?pkg=qt6-base&arch=ia64&ver=6.4.0%2Bdfsg-1&stamp=1664653669&raw=0

The different implementation available on ia64 (i.e. clone() vs clone2()) is also explained in the clone(2) man page: https://man7.org/linux/man-pages/man2/clone.2.html#NOTES