[LLD][ELF][AArch64] .note.gnu.property sections should have alignment 8

The .note.gnu.property SHT_NOTE sections on AArch64 (a 64-bit target)
should have alignment 8 to more closely match the binutils implementation
where alignment is 4-bytes on 32-bit machines and 8-bytes on 64-bit
machines.

Previously LLD was using 4 for both 32-bit and 64-bit machines.

Differential Revision: https://reviews.llvm.org/D70962