tools/nolibc: avoid false-positive -Wmaybe-uninitialized through waitpid()

The compiler does not know that waitid() will only ever return 0 or -1.
If waitid() would return a positive value than waitpid() would return that
same value and *status would not be initialized.
However users calling waitpid() know that the only possible return values
of it are 0 or -1. They therefore might check for errors with
'ret == -1' or 'ret < 0' and use *status otherwise. The compiler will then
warn about the usage of a potentially uninitialized variable.

Example:

	$ cat test.c
	#include <stdio.h>
	#include <unistd.h>

	int main(void)
	{
		int ret, status;

		ret = waitpid(0, &status, 0);
		if (ret == -1)
			return 0;

		printf("status %x\n", status);

		return 0;
	}

	$ gcc --version
	gcc (GCC) 15.1.1 20250425

	$ gcc -Wall -Os -Werror -nostdlib -nostdinc -static -Iusr/include -Itools/include/nolibc/ -o /dev/null test.c
	test.c: In function ‘main’:
	test.c:12:9: error: ‘status’ may be used uninitialized [-Werror=maybe-uninitialized]
	   12 |         printf("status %x\n", status);
	      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
	test.c:6:18: note: ‘status’ was declared here
	    6 |         int ret, status;
	      |                  ^~~~~~
	cc1: all warnings being treated as errors

Avoid the warning by normalizing waitid() errors to '-1' in waitpid().

Fixes: 0c89abf ("tools/nolibc: implement waitpid() in terms of waitid()")
Signed-off-by: Thomas Weißschuh <[email protected]>
Acked-by: Willy Tarreau <[email protected]>
Link: https://lore.kernel.org/r/20250707-nolibc-waitpid-uninitialized-v1-1-dcd4e70bcd8f@linutronix.de
Signed-off-by: Thomas Weißschuh <[email protected]>

Author: t-8ch

tools/include/nolibc/sys/wait.h

@@ -78,7 +78,7 @@ pid_t waitpid(pid_t pid, int *status, int options)
 
 	ret = waitid(idtype, id, &info, options);
 	if (ret)
-		return ret;
+		return -1;
 
 	switch (info.si_code) {
 	case 0: