trunc: Implement result sign according to IEEE 754.

trunc: Implement result sign according to IEEE 754.

* lib/trunc.c (MIN, MINUS_ZERO): New macros.
(FUNC): Return +0.0 for 0 < x < 1 and -0.0 for -1 < x < 0.
* tests/test-trunc2.c: Include minus-zero.h.
(MINUS_ZERO): New macro.
(trunc_reference): Keep in sync with lib/trunc.c.
* tests/test-truncf2.c: Include minus-zero.h.
(MINUS_ZERO): New macro.
(truncf_reference): Keep in sync with lib/trunc.c.
* tests/test-truncf-ieee.c (main): Test also values between -1 and 1.
* tests/test-trunc-ieee.c (main): Likewise.
* tests/test-truncl-ieee.c (main): Likewise.