[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);

components/libc/compilers/picolibc/syscall.c

@@ -9,13 +9,31 @@
  */
 
 #include <rtthread.h>
+#include <sys/types.h>
 
-int pico_get_errno(void)
+/* global errno */
+static volatile int __pico_errno;
+
+int *pico_get_errno(void)
 {
-    return rt_get_errno();
+    rt_thread_t tid = RT_NULL;
+
+    if (rt_interrupt_get_nest() != 0)
+    {
+        /* it's in interrupt context */
+        return &__pico_errno;
+    }
+
+    tid = rt_thread_self();
+    if (tid == RT_NULL)
+    {
+        return &__pico_errno;
+    }
+
+    return &tid->error;
 }
 
-#ifdef RT_USING_HEAP /* Memory routine */
+#ifdef RT_USING_HEAP
 void *malloc(size_t n)
 {
     return rt_malloc(n);
@@ -39,4 +57,4 @@ void free(void *rmem)
     rt_free(rmem);
 }
 RTM_EXPORT(free);
-#endif
+#endif /* RT_USING_HEAP */