Remove .syntax unified from the aarch64 common header

This directive isn't available in aarch64 code, only in arm code.
Add \() to macro arguments to separate the argument from the following .8h or similar
Add correct arguments to the gnu version of UNPACK_FILTER_SINGLE_TAG_16BITS
Use a plain "ret" instead of "ret lr"

This fixes an issue with assembling with gnu binutils.
Use the correct ext syntax in the gnu version of macros
Add macros for the non-standard mov.16b/mov.8b/ext.16b/ext.8b

This fixes building with gnu binutils, which don't support this
nonstandard form of the instructions.

Once Apple's tools support the proper standard form of the
instructions, the code should be updated to use that everywhere
instead, and these macros should be removed.
Merge pull request #738 from mstorsjo/gnu-aarch64

Fix building the aarch64 assembly using gnu binutils