[powerpc] Rename fegetenv_status to fegetenv_control

fegetenv_status is used variously to retrieve the FPSCR exception enable
bits, rounding mode bits, or both.  These are referred to as the control
bits in the POWER ISA.  FPSCR status bits are also returned by the
'mffs' and 'mffsl' instructions, but they are uniformly ignored by all
uses of fegetenv_status.  Change the name to be reflective of its
current and expected use.

Reviewed-By: Paul E Murphy <murphyp@linux.ibm.com>