Print default options selection for -march,-mcpu and -mtune for aarch64 (PR driver...

Print default options selection for -march,-mcpu and -mtune for aarch64 (PR driver/83193).

2018-08-27  Martin Liska  <mliska@suse.cz>

        PR driver/83193
	* config/aarch64/aarch64.c (aarch64_override_options_internal):
        Set default values for x_aarch64_*_string strings.
	* config/aarch64/aarch64.opt: Remove --{march,mcpu,mtune}==
        prefix.  For -mabi do not print '=ABI' in help and use
        <option_value> format for -msve-vector-bits and -moverride
        options.

From-SVN: r263870