[powerpc] fegetexcept: utilize function instead of duplicating code

fegetexcept() included code which exactly duplicates the code in
fenv_reg_to_exceptions().  Replace with a call to that function.

2019-06-05  Paul A. Clarke  <pc@us.ibm.com>

	* sysdeps/powerpc/fpu/fegetexcept.c (__fegetexcept): Replace code
	with call to equivalent function.