gdb: Fix building with latest libc++

Latest libc++[1] causes transitive include to <locale> when
<mutex> or <thread> header is included. This causes
gdb to not build[2] since <locale> defines isupper/islower etc.
functions that are explicitly macroed-out in safe-ctype.h to
prevent their use.
Use the suggestion from libc++ to include <locale> internally when
building in C++ mode to avoid build errors.
Use safe-gdb-ctype.h as the include instead of "safe-ctype.h"
to keep this isolated to gdb since rest of binutils
does not seem to use much C++.

[1]: https://reviews.llvm.org/D144331
[2]: https://issuetracker.google.com/issues/277967395

gdb/cp-name-parser.y

@@ -40,7 +40,7 @@
 #include "defs.h"
 
 #include <unistd.h>
-#include "safe-ctype.h"
+#include "gdbsupport/gdb-safe-ctype.h"
 #include "demangle.h"
 #include "cp-support.h"
 #include "c-support.h"