selftests/exec: Check if the syscall exists and bail if not

selftests/exec: Check if the syscall exists and bail if not

On systems which don't implement sys_execveat(), this test produces a
lot of output.

Add a check at the beginning to see if the syscall is present, and if
not just note one error and return.

When we run on a system that doesn't implement the syscall we will get
ENOSYS back from the kernel, so change the logic that handles
__NR_execveat not being defined to also use ENOSYS rather than -ENOSYS.

Signed-off-by: Michael Ellerman <[email protected]>
Acked-by: David Drysdale <[email protected]>
Signed-off-by: Shuah Khan <[email protected]>