_FP_ROUND_ZERO is defined as 0 and used as a statemente in macro
_FP_ROUND. This generates "error: statement with no effect
[-Werror=unused-value]" from gcc. Defining _FP_ROUND_ZERO as (void)0 to
fix it.

This modification is quoted from glibc 'commit <In libc/:>
(8ed1e7d5894000c155acbd06f)'

Signed-off-by: Vincent Chen <vincentc@andestech.com>
Acked-by: Greentime Hu <greentime@andestech.com>
Signed-off-by: Greentime Hu <greentime@andestech.com>