kselftest/arm64: Print ASCII version of unknown signal frame magic values

The signal magic values are supposed to be allocated as somewhat meaningful
ASCII so if we encounter a bad magic value print the any alphanumeric
characters we find in it as well as the hex value to aid debuggability.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20221102140543.98193-1-broonie@kernel.org


Signed-off-by: Will Deacon <will@kernel.org>