round: Implement result sign according to IEEE 754.

round: Implement result sign according to IEEE 754.

* lib/round.c (MIN, MINUS_ZERO): New macros.
(FUNC): Return -0.0 for -0.5 < x < 0.
* tests/test-roundf-ieee.c (main): Test also values between -1 and 1.
* tests/test-round-ieee.c (main): Likewise.
* tests/test-roundl-ieee.c (main): Likewise.