Get continuations working on aarch64.

* unwind.c (UW_CONT_FRAME_BEFORE, UW_CONT_FRAME_AFTER): New
preprocessor symbols.
(revive_cont): The "frame slack" zero-filled padding logic
is replaced by capturing an actual part of the real stack
before the uw_stack unwind frame.  On aarch64, there is
content there we must actually capture. Experiment shows
that exactly 128 bytes is enough, and that corresponds to
the frame_slack value.
(capture_cont): Capture UW_CONT_FRAME_BEFORE bytes before
the uw_stack unwind frame location. Also, the "capture_extra"
is replaced by UW_CONT_FRAME_AFTER constant, to harmonize.

* unwind.h (UW_FRAME_ALIGN): New preprocessor symbol.
(union uw_frame): On aarch64, we ask the compiler, via
a GCC-specific attribute syntax, to align the address of
frame objects to a 16 byte boundary. This solves a crash
in the continuation code. Continuation capture is keyed
to unwind frame addresses. When a captured continuation is
revived onto the stack, the delta between its original address
and the revive address must be a multiple of 16. The reason
is that this preserves the stack and frame pointer alignment.
Certain instructions on the aarch64, require the stack pointer
to be 16 byte aligned. There are other ways we could achieve
this, but the easiest is to align the original frames to 16
bytes.

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