powerpc: Fix llround spurious inexact on 32-bit POWER4 [BZ #22697]

This issue is similar to BZ #19235, where spurious exceptions are
created from adding 0.5 then converting to an integer.
The solution is based on Joseph's fix for BZ #19235.

	[BZ #22697]
	* sysdeps/powerpc/powerpc32/power4/fpu/s_llround.S (__llround):
	Do not add 0.5 to integer or out-of-range arguments.