write_volatile on aarch64 may not properly set ESR

[qwandor]

With the new version of LLVM in rustc 1.78.0, pointer::write_volatile can sometimes get optimised into something like str w9, [x0], #4. This writes to the MMIO memory as expected, but suppresses the syndrome register (e.g. ESR_EL1) update, which at least in the case of KVM with crosvm prevents the VMM from servicing the fault as it relies on the ESR for this.

This could arguably be considered a bug in rustc as write_volatile is explicitly intended for MMIO, but we may want to consider working around this in the meantime by using inline assembly for volatile memory access on aarch64.