aarch64: Define elf_backend_extern_protected_data to 0 [PR 18705]

Follow-up to commit 90b7a5df152a64d2bea20beb438e8b81049a5c30
("aarch64: Disallow copy relocations on protected data").

Commit 32f573bcb3aaa1c9defcad79dbb5851fcc02ae2d changed ld to produce
R_AARCH64_GLOB_DAT but that defeated the purpose of protected visibility
as an optimization.  Restore the previous behavior (which matches
ld.lld) by defining elf_backend_extern_protected_data to 0.