Don't use the non-standard apple syntax in a gnu version of a macro
Use mov.16b instead of mov.8h

According to the arm architecture reference manual, the mov (vector)
instruction can only use the arrangement specifiers '8b' and '16b'.
The apple tools still accept the '8h' form, but it assembles into the
same as '16b'. (When copying a vector register to another, the element
size in the vectors don't matter.)

This fixes building with gnu binutils.
Merge pull request #1107 from mstorsjo/fix-aarch64-gnu-build

Fix building for aarch64 with gnu binutils