CRASH aarch64 under QEMU at proc_init `mrs x0, ID_AA64MMFR2_EL1`

On Ubuntu22 (where we have to migrate: #7270), every aarch64 test run under QEMU hits a SIGILL from the final MRS instruction in proc_init: `mrs x0, ID_AA64MMFR2_EL1`.

This shows up as:
```
qemu: uncaught target signal 4 (Illegal instruction) - core dumped
```

DR only intercepts SIGSEGV and SIGBUS during init time. I'd like to intercept SIGILL and put these MRS instructions in a try-except: but it's tricky since we use SIGILL for nudges and suspending later on. I may try to add an init-time-only try-except handling.

This is blocking us from moving to Ubuntu22 on Github Actions as mandated: #7270.