raidz_test: use only async-signal-safe functions in signal handler

execl*() before glibc 2.24 could allocate, but only if called with at
least 1024 arguments, which five isn't

errno modification is also fine, so long as we restore it at the end

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #12086