alpha: fix floor on sNaN input

alpha: fix floor on sNaN input

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

Finally remove the code to handle inexact exception, floor should never
generate such an exception.

Changelog:
	* sysdeps/alpha/fpu/s_floor.c (__floor): Add argument with itself
	when it is a NaN.
	[_IEEE_FP_INEXACT] Remove.
	* sysdeps/alpha/fpu/s_floorf.c (__floorf): Likewise.

(cherry picked from commit 65cc568cf57156e5230db9a061645e54ff028a41)
(cherry picked from commit 1912cc082df4739c2388c375f8d486afdaa7d49b)