runtime/pprof: skip tests for AIX

Most of the time, the pprof tests are passing, except
for the builder. The reason is still unknown but I'd rather release
the builder to avoid missing other more important bugs.

Updates #45170

Change-Id: I667543ee1ae309b7319c5b3676a0901b4d0ecf2e
Reviewed-on: https://go-review.googlesource.com/c/go/+/306489


Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Trust: Lynn Boger <laboger@linux.vnet.ibm.com>