linux: Move xmknod{at} to compat symbols

It also decouple mknod{at} from xmknod{at}.  The riscv32 ABI was added
on 2.33, so it is safe to remove the old __xmknot{at} symbols and just
provide the newer mknod{at} ones.

Checked with a build for all affected ABIs. I also checked on x86_64,
i686, powerpc, powerpc64le, sparcv9, sparc64, s390, and s390x.

Reviewed-by: Lukasz Majewski <lukma@denx.de>