runtime: gdb isn't available on aix/ppc64

[Helflym]

gdb isn't yet available on aix/ppc64.
Therefore, tests inside runtime/runtime-gdb_test.go are skipped by https://go-review.googlesource.com/c/go/+/146939/.

This is a tracking-bug.

[gopherbot]

Change https://golang.org/cl/156478 mentions this issue: cmd/link: fix debug_frame generation for Dwarf64