[gdb/testsuite] Fix gdb.cp/break-f-std-string.cc with older gcc

[gdb/testsuite] Fix gdb.cp/break-f-std-string.cc with older gcc

When running test-case gdb.cp/break-f-std-string.exp on openSUSE Leap 15.3
with system gcc 7.5.0, I run into:
...
(gdb) whatis /r std::string^M
No symbol "string" in namespace "std".^M
(gdb) FAIL: gdb.cp/break-f-std-string.exp: _GLIBCXX_USE_CXX11_ABI=1: \
  whatis /r std::string
...
The same for gcc 8.2.1, but it passes with gcc 9.3.1.

At source level (as we can observe in the .ii file with -save-temps) we have
indeed:
...
namespace std {
  namespace __cxx11 {
    typedef basic_string<char> string;
  }
}
...
while with gcc 9.3.1, we have instead:
...
namespace std {
  namespace __cxx11 {
    ...
  }
  typedef basic_string<char> string;
}
...
due to gcc commit 33b43b0d8cd ("Define std::string and related typedefs
outside __cxx11 namespace").

Fix this by adding the missing typedef for gcc version 5 (the first version to
have the dual abi) to 8 (the last version missing aforementioned gcc commit).

Tested on x86_64-linux, with:
- system gcc 7.5.0
- gcc 4.8.5, 8.2.1, 9.3.1, 10.3.0, 11.2.1
- clang 8.0.1, 12.0.1