[libc][picolibc] fix llvm compling warning

/home/runner/work/rt-thread/rt-thread/components/libc/compilers/picolibc/syscall.c:24:16: warning: returning 'volatile int *' from a function with result type 'int *' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
        return &__pico_errno;
               ^~~~~~~~~~~~~
/home/runner/work/rt-thread/rt-thread/components/libc/compilers/picolibc/syscall.c:30:16: warning: returning 'volatile int *' from a function with result type 'int *' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
        return &__pico_errno;
               ^~~~~~~~~~~~~
/home/runner/work/rt-thread/rt-thread/components/libc/compilers/picolibc/syscall.c:33:12: warning: incompatible pointer types returning 'rt_err_t *' (aka 'long *') from a function with result type 'int *' [-Wincompatible-pointer-types]
    return &tid->error;

components/libc/compilers/picolibc/syscall.c

@@ -21,16 +21,16 @@ int *pico_get_errno(void)
     if (rt_interrupt_get_nest() != 0)
     {
         /* it's in interrupt context */
-        return &__pico_errno;
+        return (int *)&__pico_errno;
     }
 
     tid = rt_thread_self();
     if (tid == RT_NULL)
     {
-        return &__pico_errno;
+        return (int *)&__pico_errno;
     }
 
-    return &tid->error;
+    return (int *)&tid->error;
 }
 
 #ifdef RT_USING_HEAP