Fix patchable-function-entry on arc

The problem here is arc looks at current_output_insn unconditional
but sometimes current_output_insn is NULL.  With patchable-function-entry,
it will be. This is similar to how the nios2, handles "%.".

Committed as obvious after a simple test with -fpatchable-function-entry=1.

ChangeLog:
* config/arc/arc.c (output_short_suffix): Check insn for nullness.