[PATCH] CMakeLists.txt: Handle 'undefined' values returned by getconf

[PATCH] CMakeLists.txt: Handle 'undefined' values returned by getconf

This allows to avoid build failures when using qemu user mode for riscv64 [1]:
the corresponding bug in qemu [2] will get fixed but handling that here allows
to keep pocl buildable by old qemu versions.

[1]: https://bugs.launchpad.net/ubuntu/+source/pocl/+bug/1942895
[2]: https://bugs.launchpad.net/ubuntu/+source/qemu/+bug/1942884

Signed-off-by: Alexandre Ghiti <alexandre.ghiti@canonical.com>
Gbp-Pq: Name 1002-CMakeLists.txt-Handle-undefined-values-returned-by-g.patch