Restore original ifdef in limits.h for the include_next of system's l…

…imits.h

Commit 2f5f843 made changes to the
preprocessor guard that gated the include_next directive for limits.h

Currently the guard looks something like this:

```
ifdef __GNUC__
 include_next<limits.h>
endif
```

The original guard, on the other hand, also checked for _GCC_LIMITS_H_.
This extra condition is necessary to build with Clang. This error
message is shown:

> fatal error: 'limits.h' file not found

Because include_next is run when it shouldn't.

newlib/libc/include/limits.h

@@ -131,13 +131,14 @@
 /*
  * Placing this outside of the above condition means that this will
  * get run even from another picolibc provided limits.h file down the
- * include chain.
+ * include chain. This also to see if the compiler limits.h has
+ * already been included as some clang configurations do that.
  */
-#ifdef __GNUC__
+#if defined __GNUC__ && !defined _GCC_LIMITS_H_
 #ifdef __clang__
 #ifndef __GLIBC_USE
 #define __GLIBC_USE(x) 1
 #endif
 #endif
 # include_next <limits.h>
-#endif /* __GNUC__ */
+#endif /* __GNUC__ && !_GCC_LIMITS_H_ */