libc: tmpfile shouldn't hardcode the folder to /tmp

use P_tmpdir macro instead

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>

libs/libc/stdio/lib_tmpfile.c

@@ -32,7 +32,7 @@
 
 FAR FILE *tmpfile(void)
 {
-  char path[L_tmpnam] = "/tmp/XXXXXX.tmp";
+  char path[L_tmpnam] = P_tmpdir "/XXXXXX.tmp";
   FAR FILE *fp = NULL;
   int fd;