gdb/nat/linux-osdata.c: fix build on gcc-12 (string overfow)

gdb/nat/linux-osdata.c: fix build on gcc-12 (string overfow)

On gcc-12 build fails as:

    ../../gdbserver/../gdb/nat/linux-osdata.c: In function 'void linux_xfer_osdata_processes(buffer*)':
    ../../gdbserver/../gdb/nat/linux-osdata.c:330:39: error:
      '__builtin___sprintf_chk' may write a terminating nul past the end of the destination [-Werror=format-overflow=]
      330 |                 sprintf (core_str, "%d", i);
          |                                       ^

It's an off-by-one case in an infeasible scenario for negative
huge core count. The change switches to std::string for memory
handling.

Tested by running 'info os processes' and checking CPU cores column.