cmd/internal/obj/ppc64: add a test for long branch fixups
cmd/internal/obj/ppc64: add a test for long branch fixups

Cribbed and modified from arm64, verify each transformation
rewrites a too-far conditional branch as expected.

Change-Id: I87d35085158ed7d7478aa9725b273401fcd0bd01
Reviewed-on: https://go-review.googlesource.com/c/go/+/347049
Trust: David Chase <ae171625adc8645b8b79b7e405a6da5aba5341e0@google.com>
Reviewed-by: Lynn Boger <b27445fe6fb259c37b8e0a4ce56e22e2413754bf@linux.vnet.ibm.com>

golang/go,nawawi/go,CAFxX/go,nawawi/go,CAFxX/go,golang/go,nawawi/go,cloudflare/go,nawawi/go,cloudflare/go,cloudflare/go,nawawi/go,nawawi/go,CAFxX/go,cloudflare/go,cloudflare/go,CAFxX/go,golang/go,cloudflare/go,CAFxX/go,cloudflare/go,CAFxX/go,golang/go,golang/go,cloudflare/go,nawawi/go,golang/go,golang/go,CAFxX/go,nawawi/go,golang/go,CAFxX/go