m68k: Enable dead code elimination

Allow the experimental dead code elimination config to be enabled.

For my 68000 nommu config this frees up a few hundred K of memory
so seems worth while.

Boot and build tested on nommu and mmu enabled configs.

Before:
Memory: 5388K/8192K available (1986K kernel code, 114K rwdata,
244K rodata, 92K init, 41K bss, 2624K reserved, 0K cma-reserved)

After
Memory: 5684K/8192K available (1714K kernel code, 112K rwdata,
228K rodata, 92K init, 37K bss, 2328K reserved, 0K cma-reserved)

Signed-off-by: Daniel Palmer <daniel@0x0f.com>
Acked-by: Greg Ungerer <gerg@linux-m68k.org>
Tested-by: Finn Thain <fthain@linux-m68k.org>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/20250416114240.2929832-1-daniel@0x0f.com


Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>