Fallback to strerror() when strerror_l() isn't available

Some C libraries, such as uClibc, do not provide strerror_l() in
which case we fallback to strerror().

Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #16636
Closes #16640