x86/tools: insn_decoder_test.c: Emit standard build success messages

The standard 'success' output of insn_decoder_test spams build logs with:

  arch/x86/tools/insn_decoder_test: success: Decoded and checked 8258521 instructions

Prefix the message with the standard '  ' (two spaces) used by kbuild to denote
regular build messages, making it easier for tools to filter out
warnings and errors.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Jürgen Groß <jgross@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Michal Marek <michal.lkml@markovi.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://lore.kernel.org/r/20250515132719.31868-5-mingo@kernel.org

Author: Ingo Molnar

arch/x86/tools/insn_decoder_test.c

@@ -167,7 +167,7 @@ int main(int argc, char **argv)
 		pr_warn("Decoded and checked %d instructions with %d "
 			"failures\n", insns, warnings);
 	else
-		fprintf(stdout, "%s: success: Decoded and checked %d"
+		fprintf(stdout, "  %s: success: Decoded and checked %d"
 			" instructions\n", prog, insns);
 	return 0;
 }