selftests/x86/syscall: Simplify message reporting in syscall_numbering

Reduce some boiler plate in printing and indenting messages.
This makes it easier to produce clean status output.

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20210518191303.4135296-3-hpa@zytor.com