test: workaround SIGILL on issue11656 on aix

test: workaround SIGILL on issue11656 on aix

For some reason, aix sometimes executes the bogus function body. This
should never happen as it lives in a no-execute section. It might be
a transient permission blip as the heap grows.

Add a small function to cleanup and synchronize the icache before
jumping to the bogus function to ensure it causes a panic, not SIGILL.

Fixes #44583

Change-Id: Iadca62d82bfb70fc62088705dac42a880a1208fa
Reviewed-on: https://go-review.googlesource.com/c/go/+/377314
Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Paul Murphy <murp@ibm.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>