ARC: .exit.* sections can be discarded in .eh_frame regime

We used to keep the .exit.* sections as linker would fail in final link
due to references from .debug_frame which itself could not be discardrd
due to the forced "write,alloc" attributes for it.

|   LD      init/built-in.o
| `.exit.text' referenced in section `.debug_frame' of arch/arc/built-in.o: defined in discarded section `.exit.text' of arch/arc/built-in.o
| Makefile:949: recipe for target 'vmlinux' failed

With .debug_frame now retired, this hack is no longer needed.
kernel binary is now a little bit smaller as well.

closes STAR 9000549913

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>