alpha: fix rint on sNaN input

The alpha version of rint wrongly return sNaN for sNaN input. Fix that
by checking for NaN and by returning the input value added with itself
in that case.

Changelog:
	* sysdeps/alpha/fpu/s_rint.c (__rint): Add argument with itself
	when it is a NaN.
	* sysdeps/alpha/fpu/s_rintf.c (__rintf): Likewise.

(cherry picked from commit cb7f9d63)