floor: Implement result sign according to IEEE 754.

floor: Implement result sign according to IEEE 754.

* lib/floor.c (FUNC): Return +0.0 for 0 < x < 1.
* tests/test-floor2.c (floor_reference): Keep in sync with lib/floor.c.
* tests/test-floorf2.c (floorf_reference): Likewise.
* tests/test-floorf-ieee.c (main): Test also values between -1 and 1.
* tests/test-floor-ieee.c (main): Likewise.
* tests/test-floorl-ieee.c (main): Likewise.