gdbserver-amd64: add HAVE_STRUCT_USER_REGS_STRUCT_(GS|FS)_BASE for gdbserver.

The macros mentioned in the title were set only for GDB. In gdbserver they
were not set until now.  To align the code in GDB and gdbserver these macros
are also added into gdbserver, enabling read and write of gs_base and fs_base
registers from the system in new and old kernels.

2017-01-27  Walfred Tedeschi  <walfred.tedeschi@intel.com>

gdb/gdbserver/ChangeLog:

    	* configure.ac: Check if the fs_base and gs_base members of
    	`struct user_regs_struct' exist.
    	* config.in: Regenerated.
    	* configure: Likewise.