Update crt0.S for ARC.

Update crt0.S for ARC.

This is similar to commit 06537f05d4b6a0d2db01c6afda1d2a0ea2588126 to the
newlib for ARC.

GCC for ARC has been updated to provide consistent naming of preprocessor
definitions for different optional architecture features:

    * __ARC_BARREL_SHIFTER__ instead of __Xbarrel_shifter for
      -mbarrel-shifter
    * __ARCEM__ instead of __EM__ for ARC EM cores
    * __ARCHS__ instead of __HS__ for ARC HS cores
    * etc (not used in libgloss)

This patch updates crt0.S for ARC to use new definitions instead of a
deprecated ones. To ensure compatibility with older compiler new definitions
are also defined in crt0.S if needed, based on presence of deprecated
preprocessor definitions.

libgloss/ChangeLog

2016-04-29  Anton Kolesov  <Anton.Kolesov@synopsys.com>

        * arc/crt0.S: Use new GCC defines to detect processor features.