[gdb/build] Fix build with -std=c++11

When building with -std=c++11, we run into two Werror=missing-declarations:
...
new-op.cc: In function 'void operator delete(void*, std::size_t)':
new-op.cc:114:1: error: no previous declaration for \
  'void operator delete(void*, std::size_t)' [-Werror=missing-declarations]
 operator delete (void *p, std::size_t) noexcept
 ^~~~~~~~
new-op.cc: In function 'void operator delete [](void*, std::size_t)':
new-op.cc:132:1: error: no previous declaration for \
  'void operator delete [](void*, std::size_t)' [-Werror=missing-declarations]
 operator delete[] (void *p, std::size_t) noexcept
 ^~~~~~~~
...

These are due to recent commit 5fff6115fe "Fix
LD_PRELOAD=/usr/lib64/libasan.so.6 gdb".

The declarations are provided by <new> (which is included) for c++14 onwards,
but they are missing for c++11.

Fix this by adding the missing declarations.

Tested on x86_64-linux, with gcc 7.5.0, both without (implying -std=gnu++14) and
with -std=c++11.

gdbsupport/new-op.cc

@@ -27,6 +27,11 @@
 #include "host-defs.h"
 #include <new>
 
+/* These are declared in <new> starting C++14.  Add these here to enable
+   compilation using C++11. */
+extern void operator delete (void *p, std::size_t) noexcept;
+extern void operator delete[] (void *p, std::size_t) noexcept;
+
 /* Override operator new / operator new[], in order to internal_error
    on allocation failure and thus query the user for abort/core
    dump/continue, just like xmalloc does.  We don't do this from a