runtime/pprof: implement pprof for aix/ppc64

[Helflym]

pprof tool isn't yet implemented for aix/ppc64.
Therefore, https://go-review.googlesource.com/c/go/+/146939 disables TestTimePprof for aix/ppc64.

This is a tracking bug.

[gopherbot]

Change https://golang.org/cl/164013 mentions this issue: runtime: disable pprof test with cgo on aix/ppc64

[gopherbot]

Change https://golang.org/cl/169117 mentions this issue: runtime: improve sigtramp on aix/ppc64 to handle SIGPROF