testsuite: Add -mtune=generic to dg-options for two testcases.

Use -mtune=generic to limit these two test cases. Because configuring them with
-mtune=cascadelake or znver3 will vectorize them.

gcc/testsuite/ChangeLog:

	* gcc.dg/vect/costmodel/x86_64/costmodel-pr104582-2.c: Add
	-mtune=generic to dg-options.
	* gcc.target/i386/pr84101.c: Likewise.