[PATCH][AArch64]Fix the problem that the arm_neon.h can't be used in a cpp file

[Tim Northover]

Hi Hao,

> A simple patch to fix the problem about using arm_neon.h  in .cpp file will
> trigger an assertion failure. As the one element vector is indeed used in
> aarch64, the assertion is unnecessary and wrong.

This looks reasonable, given what we've got at the moment. Longer term
we may want to look into completely merging poly64x1_t and poly64_t,
but either choice seems to be within ACLE.

Cheers.

Tim.