libstdc++: Fix warnings with -Wsystem-headers

libstdc++-v3/ChangeLog:

	* include/bits/stl_algobase.h (__find_if): Add FALLTHRU markers.
	* include/std/charconv (__detail::__to_chars): Avoid
	-Wsign-compare warning.

(cherry picked from commit 25920dd18ad12ea501309b1487366e22f35db631)

libstdc++-v3/include/bits/stl_algobase.h

@@ -1952,14 +1952,17 @@ _GLIBCXX_END_NAMESPACE_ALGO
 	  if (__pred(__first))
 	    return __first;
 	  ++__first;
+	  // FALLTHRU
 	case 2:
 	  if (__pred(__first))
 	    return __first;
 	  ++__first;
+	  // FALLTHRU
 	case 1:
 	  if (__pred(__first))
 	    return __first;
 	  ++__first;
+	  // FALLTHRU
 	case 0:
 	default:
 	  return __last;

libstdc++-v3/include/std/charconv

@@ -142,7 +142,7 @@ namespace __detail
 	'u', 'v', 'w', 'x', 'y', 'z'
       };
 
-      while (__val >= __base)
+      while (__val >= (unsigned)__base)
 	{
 	  auto const __quo = __val / __base;
 	  auto const __rem = __val % __base;