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

gdbsupport/gdb-safe-ctype.h

@@ -23,7 +23,9 @@
 /* After safe-ctype.h is included, we can no longer use the host's
    ctype routines.  Trying to do so results in compile errors.  Code
    that uses safe-ctype.h that wants to refer to the locale-dependent
-   ctype functions must call these wrapper versions instead.  */
+   ctype functions must call these wrapper versions instead.
+   When compiling in C++ mode, also include <locale> before "safe-ctype.h"
+   which also defines is* functions.  */
 
 static inline int
 gdb_isprint (int ch)
@@ -41,6 +43,7 @@ gdb_isprint (int ch)
 #undef ISUPPER
 #undef ISXDIGIT
 
+#include <locale>
 #include "safe-ctype.h"
 
 #endif