[llvm-exegesis] Preserve rcx and r11 around system call (#72807)

Currently, when making the ioctl system call, we're not preserving rcx
and r11. The system call will clobber these registers, meaning that the
values of the registers in the snippet will be different than expected.
This patch fixes that be preserving the registers around the system
call, similar to how the other registers involved in the making the
system call get preserved.

Fixes #72741.