This commit fixes backtrace if a crash or an exit signal is received
during a C syscall on aix/ppc64.
This is similar to Solaris, Darwin or Windows implementation.

Change-Id: I6040c0b1577a9f5b298f58bd4ee6556258a135ef
Reviewed-on: https://go-review.googlesource.com/c/154718
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>