gdbsupport: fix array-view compilation with c++11 && _GLIBCXX_DEBUG

gdbsupport: fix array-view compilation with c++11 && _GLIBCXX_DEBUG

When building with -std=c++11 and -D_GLIBCXX_DEBUG=1, we get some errors
like:

      CXX    unittests/array-view-selftests.o
    In file included from /home/smarchi/src/binutils-gdb/gdb/utils.h:25,
                     from /home/smarchi/src/binutils-gdb/gdb/defs.h:630,
                     from /home/smarchi/src/binutils-gdb/gdb/unittests/array-view-selftests.c:20:
    /home/smarchi/src/binutils-gdb/gdb/../gdbsupport/array-view.h: In instantiation of constexpr gdb::array_view<T> gdb::array_view<T>::slice(gdb::array_view<T>::size_type, gdb::array_view<T>::size_type) const [with T = unsigned char; gdb::array_view<T>::size_type = long unsigned int:
    /home/smarchi/src/binutils-gdb/gdb/unittests/array-view-selftests.c:532:29:   required from here
    /home/smarchi/src/binutils-gdb/gdb/../gdbsupport/array-view.h:192:3: error: body of constexpr function constexpr gdb::array_view<T> gdb::array_view<T>::slice(gdb::array_view<T>::size_type, gdb::array_view<T>::size_type) const [with T = unsigned char; gdb::array_view<T>::size_type = long unsigned int not a return-statement
      192 |   }
          |   ^

This is because constexpr functions in c++11 can only consist of a
single return statement, so we can't have the gdb_assert in there.  Make
the gdb_assert presence conditional to the __cplusplus version, to
enable it only for c++14 and later.

Change-Id: I2ac33f7b4bd1765ddc3ac8d07445b16ac1f340f0