util/getauxval: Ensure setting errno if not found

util/getauxval: Ensure setting errno if not found

Sometimes zero is a valid value for getauxval (e.g. AT_EXECFD). Make
sure that we can distinguish between a valid zero value and a not found
entry by setting errno.

Assumes that getauxval from sys/auxv.h sets errno correctly.

Signed-off-by: Vivian Wang <uwu@dram.page>
Message-ID: <20240723100545.405476-2-uwu@dram.page>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>