arc: Use @pcl assembler syntax instead of invalid expressions

arc: Use @pcl assembler syntax instead of invalid expressions

The old ARC assembler would accept expressions like 'LABEL-(.&-4)'
which would calculate the offset from the PCL to LABEL.  The new ARC
assembler does not accept these expressions, instead there's an @pcl
synax, used like LABEL@pcl which gives the offset from PCL to LABEL.

Most of the use of the old expression syntax have been removed,
however, this one got missed.

gcc/ChangeLog:

	* config/arc/arc.md (doloop_begin_i): Use @pcl assembler syntax.

From-SVN: r246932