runtime: run TestCgoExternalThreadSIGPROF on ppc64le

It was disabled because of the lack of external linking.

Change-Id: Iccb4a4ef8c57d048d53deabe4e0f4e6b9dccce33
Reviewed-on: https://go-review.googlesource.com/16797


Reviewed-by: Ian Lance Taylor <iant@golang.org>