kselftest/arm64: Log fp-stress child startup errors to stdout

kselftest/arm64: Log fp-stress child startup errors to stdout

Currently if we encounter an error between fork() and exec() of a child
process we log the error to stderr. This means that the errors don't get
annotated with the child information which makes diagnostics harder and
means that if we miss the exit signal from the child we can deadlock
waiting for output from the child. Improve robustness and output quality
by logging to stdout instead.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20241023-arm64-fp-stress-exec-fail-v1-1-ee3c62932c15@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>