selftests: arm64: Check fread return value in exec_target

Fix -Wunused-result warning generated when compiled with gcc 13.3.0,
by checking fread's return value and handling errors, preventing
potential failures when reading from stdin.

Fixes compiler warning:
warning: ignoring return value of 'fread' declared with attribute
'warn_unused_result' [-Wunused-result]

Fixes: 806a15b2 ("kselftests/arm64: add PAuth test for whether exec() changes keys")

Signed-off-by: Bala-Vignesh-Reddy <reddybalavignesh9979@gmail.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>