i386: Sync tune_string with arch_string for target attribute arch=*

i386: Sync tune_string with arch_string for target attribute arch=*

For function with target attribute arch=*, current logic will set its
tune to -mtune from command line so all target_clones will get same
tuning flags which would affect the performance for each clone. Override
tune with arch if tune was not explicitly specified to get proper tuning
flags for target_clones.

gcc/ChangeLog:

	* config/i386/i386-options.cc (ix86_valid_target_attribute_tree):
	Override tune_string with arch_string if tune_string is not
	explicitly specified.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/mvc17.c: New test.