runtime: don't unwind past asmcgocall

asmcgocall switches to the system stack and aligns the SP, so
gentraceback both can't unwind over it when it appears on the system
stack (it'll read some uninitialized stack slot as the return PC).
There's also no point in unwinding over it, so don't.

Updates #23576.

Change-Id: Idfcc9599c7636b80dec5451cb65ae892b4611981
Reviewed-on: https://go-review.googlesource.com/90895


Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>