[1/n] remove gcc.dg/vect special naming in favor of dg-additional-options

This kicks off removal of keying options used on testcase names as
done in gcc.dg/vect as the appropriate way to do this is using
dg-additional-options.

Starting with two of the least used ones.

This causes the moved tests to be covered by VECT_ADDITIONAL_FLAGS
processing.

	* gcc.dg/vect/vect.exp: Process no-fast-math-* and
	no-math-errno-* in the main set.
	* gcc.dg/vect/no-fast-math-vect16.c: Add -fno-fast-math.
	* gcc.dg/vect/no-math-errno-slp-32.c: Add -fno-math-errno.
	* gcc.dg/vect/no-math-errno-vect-pow-1.c: Likewise.