[AArch64] Fix extern protected data handling

[AArch64] Fix extern protected data handling

Emit *GLOB_DAT instead of *RELATIVE relocs for protected data
in shared objects.

This is needed for the fix of glibc bug 17711:
https://sourceware.org/bugzilla/show_bug.cgi?id=17711

bfd:
	PR ld/18705
	* elfnn-aarch64.c (elf_backend_extern_protected_data): Define.

ld/testsuite:
	PR ld/18705
	* ld-aarch64/protected-data.d: New.
	* ld-aarch64/protected-data.s: New.
	* ld-aarch64/aarch64-elf.exp: Add new test.