fixup abi manip
aarch64: Move return value handling into ffi_closure_SYSV

As with the change to ffi_call_SYSV, this avoids copying data
into a temporary buffer.
aarch64: Unify scalar fp and hfa handling

Since an HFA of a single element is exactly the same as scalar,
this tidies things up a bit.
aarch64: Remove aarch64_flags

This field was useless from the start, since the normal flags
field is available for backend use.
aarch64: Add support for complex types