Move arch word size detection to setup_arch.h
README: bug #28831 no longer applies
Support aarch64 NEON
test: Split shuffle_generic.cc into two files.
Return const reference from expr_vec_make_const

This works around GCC inability to properly optimize out return by value
on some cases on ARM.
make_const: Don't explicitly use vldX

This is a workaround for GCC due to its inability to do constant folding
properly.