The implementation of runtime.abort on arm64 currently branches to
address 0, which results in a signal from PC 0, rather than from
runtime.abort, so the runtime fails to recognize it as an abort.

Fix runtime.abort on arm64 to read from address 0 like what other
architectures do and recognize this in the signal handler.

Should fix the linux/arm64 build.

Change-Id: I960ab630daaeadc9190287604d4d8337b1ea3853
Reviewed-on: https://go-review.googlesource.com/99895
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>