Makes AArch64 assembly acceptable to clang

According to ARMv8 architecture reference manual section C7.2.188, SIMD MOV (to
general) instruction format is

  MOV <Xd>, <Vn>.D[<index>]

gas appears to accept "<Vn>.2D[<index>]" as well, but clang's assembler does
not. C.f. https://community.arm.com/developer/ip-products/processors/f/cortex-a-forum/5214/aarch64-assembly-syntax-for-armclang