[gdb/testsuite] Add save_target_board_info

Add a proc save_target_board_info, similar to save_vars, such that we can do:
...
save_target_board_info { multilib_flags } {
    global board
    set board [target_info name]
    unset_board_info multilib_flags
    set_board_info multilib_flags "$override_multilib_flags"
    ...
}
...
and use it in gdb_compile_shlib.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-12-20  Tom de Vries  <tdevries@suse.de>

	* lib/gdb.exp (save_target_board_info): New proc.
	(gdb_compile_shlib): Use save_target_board_info.