nproc: Prefer sys/cdefs.h __unreachable over the builtin

The __builtin_unreachable macro provided by Clang and GCC is a hint to
the compiler used for optimization. The programs work fine even if the
compiler doesn't support it. The sys/cdefs.h has had __unreachable for
9 years (commit 732b31de5d). It expands
to the builtin if it is available. In the rare case that it is
unsupported it expands to a null statement so compilation does not
fail.

Signed-off-by: Collin Funk <collin.funk1@gmail.com>
Reviewed by: imp, freebsd@igalic.co
Pull Request: https://github.com/freebsd/freebsd-src/pull/1117

bin/nproc/nproc.c

@@ -94,7 +94,7 @@ main(int argc, char *argv[])
 			break;
 		case OPT_VERSION:
 			version();
-			__builtin_unreachable();
+			__unreachable();
 		case OPT_HELP:
 			help();
 			exit(EXIT_SUCCESS);