ARC: Replace homegrown code with generic skip_prologue_using_sal

Use generic skip_prologue_using_sal function instead of the homegrown code to skip prologue using symbol information. Previous implementation not only duplicated existing functionality but also was buggy, namely it couldn't handle situation when there is no prologue, which is possible with optimized code, in this situation function was skipping first line of function body.

This resolves at least one set of regression tests: gdb.opt/inline-cmds.exp. I haven't tried others.