xtensa: Make use of ROUND.S instruction

xtensa: Make use of ROUND.S instruction

Due to inconsistencies in the behavior of rounding half, making this
machine instruction available was retracted in a previous commit (5f3b5b0616fe883e86e95d9476371cf87059ca7f),
but it may be useful to have it available if strict implementation of
floating-point arithmetic is not required.

gcc/ChangeLog:

	* config/xtensa/xtensa.md
	(c_enum "unspec", int_iterator ANY_ROUND): Add UNSPEC_ROUND.
	(int_attr m_round): Add a pair of UNSPEC_ROUND and "round".
	(int_attr c_round): New integer iterator attribute, that expands
	to "flag_unsafe_math_optimizations" in the case of UNSPEC_ROUND,
	and to "1" otherwise.
	(l<m_round>sfsi2, *l<m_round>sfsi2_2x, *l<m_round>sfsi2_scaled):
	Append " && <c_round>" to the conditions.