Every resource I have found about the aarch64 ABI over Linux mentions that x8 holds the syscall number and that the arguments go in registers x0 through x5. They also note that this registers are caller-saved (including also x6 and x7) when calling subroutines (with x0 used for the returned value), but they never clarify if the kernel may or may not clobber any register apart from x0. Or maybe it isn't even the kernel, like the case with rcx and r11 in x86_64.

The syscall manpage doesn't help much:

Note that these tables don't cover the entire calling convention —some architectures may indiscriminately clobber other registers not listed here.

Well, in that same document x1 is shown as a 2nd returned value, but in x86_64 rdx is also shown as so, and, having a lot of experience with this ISA, I have never seen a syscall return anything in or clobber rdx.

Anyway, what I have seen so far (and I have very little experience with ARM/ARM64) is that the only clobbered register is x0.