[ELF][AArch64] Add aarch64_elf64_le_vec emulation

Android AOSP has started specifying -m aarch64_elf64_le_vec as supported
by gold and BFD. This is a simple change to add the emulation so that LLD
doesn't immediately error when used as a linker in an AOSP build.

Differential Revision: https://reviews.llvm.org/D46429

llvm-svn: 331521