libstdc++: Avoid try-catch and O(N) size in std::list::merge for old ABI

The current std::list::merge code calls size() before starting to merge
any elements, so that the _M_size members can be updated after the merge
finishes. The work is done in a try-block so that the sizes can still be
updated in an exception handler if any element comparison throws.

The _M_size members only exist for the cxx11 ABI, so the initial call to
size() and the try-catch are only needed for that ABI. For the old ABI
the size() call performs an O(N) list traversal to get a value that
isn't even used, and catching exceptions just to rethrow them isn't
needed either.

In r11-10123 this code was refactored to use an RAII guard type, but for
the gcc-10 branch a less invasive change using preprocessor conditionals
seems more appropriate.

libstdc++-v3/ChangeLog:

	* include/bits/list.tcc (list::merge) [!USE_CXX11_ABI]: Remove
	call to size() and try-catch block.

libstdc++-v3/include/bits/list.tcc

@@ -406,8 +406,10 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
 	  iterator __last1 = end();
 	  iterator __first2 = __x.begin();
 	  iterator __last2 = __x.end();
+#if _GLIBCXX_USE_CXX11_ABI
 	  const size_t __orig_size = __x.size();
 	  __try {
+#endif
 	    while (__first1 != __last1 && __first2 != __last2)
 	      if (*__first2 < *__first1)
 		{
@@ -422,6 +424,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
 
 	    this->_M_inc_size(__x._M_get_size());
 	    __x._M_set_size(0);
+#if _GLIBCXX_USE_CXX11_ABI
 	  }
 	  __catch(...)
 	    {
@@ -430,6 +433,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
 	      __x._M_set_size(__dist);
 	      __throw_exception_again;
 	    }
+#endif
 	}
     }