ARC: unwind: Use built-in sort swap to reduce code size and improve performance

The custom swap function used in sort() was identical to the default
built-in sort swap. Remove the custom swap function and passes NULL to
sort(), allowing it to use the default swap function.

This change reduces code size and improves performance, particularly when
CONFIG_MITIGATION_RETPOLINE is enabled. With RETPOLINE mitigation, indirect
function calls incur significant overhead, and using the default swap
function avoids this cost.

$ ./scripts/bloat-o-meter ./unwind.o.old ./unwind.o.new
add/remove: 0/1 grow/shrink: 0/1 up/down: 0/-22 (-22)
Function                                     old     new   delta
init_unwind_hdr.constprop                    544     540      -4
swap_eh_frame_hdr_table_entries               18       -     -18
Total: Before=4410, After=4388, chg -0.50%

Signed-off-by: Yu-Chun Lin <eleanor15x@gmail.com>
Signed-off-by: Vineet Gupta <vgupta@kernel.org>