Add AArch64 vrnd*_f64 Neon intrinsics.

The LLVM intrinsic doesn't support float64x1_t, but the required
instruction is a scalar form (e.g. `frint32x <Dd>, <Dn>`), so we can
implement these using the scalar intrinsic.

Note that Clang does not support these intrinsics, so they aren't
covered by intrinsic-test. Additional validation is included in this
patch to ensure that we're selecting an instruction with the same
behaviour as the corresponding vector form (which all have
intrinsic-tests).

Author: jacobbramley