libc: picolibc: fputc should return the value it has written

According to the documentations:
- en.cppreference.com/w/c/io/fputc
- pubs.opengroup.org/onlinepubs/9699919799/functions/fputc.html

Return value
- On success, returns the written character.
- On failure, returns EOF and sets the error indicator
  (see ferror()) on stream.

This commit changes it to return the written value on success,
and EOF if failed, which is the same as minimal libc, but the
error indicator is still not set.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>

Author: ycsin

lib/libc/picolibc/libc-hooks.c

@@ -30,8 +30,7 @@ static LIBC_DATA int (*_stdout_hook)(int);
 
 int z_impl_zephyr_fputc(int a, FILE *out)
 {
-	(*_stdout_hook)(a);
-	return 0;
+	return ((out == stdout) || (out == stderr)) ? (*_stdout_hook)(a) : EOF;
 }
 
 #ifdef CONFIG_USERSPACE
@@ -44,8 +43,7 @@ static inline int z_vrfy_zephyr_fputc(int c, FILE *stream)
 
 static int picolibc_put(char a, FILE *f)
 {
-	zephyr_fputc(a, f);
-	return 0;
+	return zephyr_fputc(a, f);
 }
 
 static LIBC_DATA FILE __stdout = FDEV_SETUP_STREAM(picolibc_put, NULL, NULL, 0);