tools.c: do not use keywords 'nullptr' as a variable in code

Without the patch:

  tools.c: In function ‘drop_core’:
  tools.c:6251:23: error: expected identifier or ‘(’ before ‘nullptr’
   6251 |         volatile int *nullptr;
        |                       ^~~~~~~
  tools.c:6259:17: error: lvalue required as left operand of assignment
   6259 |         nullptr = NULL;
        |                 ^
  tools.c:6261:21: error: invalid type argument of unary ‘*’ (have ‘typeof (nullptr)’)
   6261 |                 i = *nullptr;
        |                     ^~~~~~~~
  make[6]: *** [Makefile:345: tools.o] Error 1

Note: this was observed on gcc version 15.0.1

Signed-off-by: Lianbo Jiang <[email protected]>

Author: lian-bo

tools.c

@@ -6248,17 +6248,17 @@ lowest_bit_long(ulong val)
 void
 drop_core(char *s)
 {
-	volatile int *nullptr;
+	volatile int *ptr;
 	int i ATTRIBUTE_UNUSED;
 
 	if (s && ascii_string(s))
 		fprintf(stderr, "%s", s);
 
 	kill((pid_t)pc->program_pid, 3);
 
-	nullptr = NULL;
+	ptr = NULL;
 	while (TRUE)
-		i = *nullptr;
+		i = *ptr;
 }