Always use wordsize-64 version of s_rint.c.

Always use wordsize-64 version of s_rint.c.

This patch replaces s_rint.c in sysdeps/dbl-64 with the one in
sysdeps/dbl-64/wordsize-64 and removes the latter one.
The code is not changed except changes in code style.

Also adjusted the include path in x86_64 file.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>