[picolibc] fix the errno declaration conflict

  /home/runner/work/rt-thread/rt-thread/components/libc/compilers/picolibc/syscall.c:13:5: error: conflicting types for 'pico_get_errno'
  int pico_get_errno(void)
      ^
  /opt/LLVMEmbeddedToolchainForArm-16.0.0-Linux-x86_64/bin/../lib/clang-runtimes/arm-none-eabi/armv7em_hard_fpv4_sp_d16/include/sys/errno.h:59:6: note: previous declaration is here
  int *__PICOLIBC_ERRNO_FUNCTION(void);

Author: mysterywolf

components/libc/compilers/picolibc/syscall.c

@@ -10,9 +10,17 @@
 
 #include <rtthread.h>
 
-int pico_get_errno(void)
+int *pico_get_errno(void)
 {
-    return rt_get_errno();
+    rt_thread_t tid = RT_NULL;
+
+    tid = rt_thread_self();
+    if (tid)
+    {
+        return &tid->error;
+    }
+
+    return RT_NULL;
 }
 
 #ifdef RT_USING_HEAP /* Memory routine */