[compiler-rt] [compiler-rt][test] Use configured Python for lit page-size probe (PR #209175)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

This is breaking running compiler-rt tests downstream.  Example failure: https://github.com/qualcomm/cpullvm-toolchain/actions/runs/29563885868/job/87832057435 .  The failing tests are running on a qemu-emulated device.

My theory is that before this patch, the command just immediately failed: we try to run qemu with `-device loader,file=python3`, and that fails because there is no such file.  But `-device loader,file=/usr/bin/python3` finds the file... and hangs because the emulated system ends up in some kind of infinite loop.

Does that make sense?  Should we skip the target_page_size() check if tests aren't running on the host?

https://github.com/llvm/llvm-project/pull/209175