ld: Properly display default for -z separate-code
Check DEFAULT_LD_Z_SEPARATE_CODE to display default for -z separate-code.
PR ld/23358
* lexsup.c (elf_shlib_list_options): Properly display default
for -z separate-code.
(cherry picked from commit b14b7ff14ffe911ec562004304841cf7d2775a19)