arm/backtrace_fp: fix build warning
common/arm_backtrace_fp.c: In function 'up_backtrace':
common/arm_backtrace_fp.c:126:23: warning: assignment to 'void *' from 'uintptr_t' {aka 'unsigned int'} makes pointer from integer without a cast [-Wint-conversion]
126 |           istacklimit = arm_intstack_top();
|                       ^
@@ -123,7 +123,7 @@ int up_backtrace(struct tcb_s *tcb,
{
#if CONFIG_ARCH_INTERRUPTSTACK > 7
# ifdef CONFIG_SMP
- istacklimit = arm_intstack_top();
+ istacklimit = (void *)arm_intstack_top();
# else
istacklimit = g_intstacktop;
# endif /* CONFIG_SMP */
